Torque converter Noise at lowish revs

[X5MO]

My car has always averaged 26mpg urban driving according to the onboard screen. About a month ago it dropped to around 21.7mpg which I noticed but wasnt concerned about however more recently I have noticed a very slight noise which sounds as if it could be metal on metal for want of a better description. It is from the gearbox and having read an article online it is a possible symptom of the torque converter having issues.
I bought the car in January and had my local Indy check the car thoroughly including the gearbox being flushed and refilled as a matter of course due to the mileage. There are no apparent issues with the cars drive AT ALL just that when I drive off and the revs are between 1200 to 2000 I notice the sound I have mentioned which wasnt there a month ago. It only seems to happen when acceleration is applied more spiritedly and doesn't present itself in higher gears (4th 5th 6th).

I have family members who are mechanics so I have dropped the car off for a diagnosis. I am aware that there are no fault codes present.
I will update thread tomorrow and I am going to read up on the torque converter etc

Any ideas other than I need my hearing checking ? Thanks in advance

[X5Sport]

Could be the torque converter beginning to fail.  It should throw a fault code in the gearbox error system.  Not all code readers can interrogate that.  It may not bring a fault light on at all.  Mine didn't until too late.....the gearbox had already digested itself!!

Richard

[X5MO]

Thanks for that Richard , just had a phone call and the gearbox oil is apparently a murky brown colour showing signs of exposure to excessive heat. They have drained it and removed the torque converter which will be on its way to sussex autoparts for reconditioning if its not too badly damaged.
I will update further once I have a report from sussex autoparts. I have been asked to get some ATF cooler line flush to remove deposits there and in the cooler. I cant find anything in the UK. I have seen an article that advises the use of brake cleaner in the return line and leaving it in there for a short time and then using an air line set at no more than 15psi ?

Lots to think about and i'm still not sure if the gearbox is damaged, How would I know for sure ? Visually it looks okay
MO

[X5Sport]

New torque converters from ZF are about £250 and if the oil is brown and smells burnt then it may also be a sign of damage on the clutch and brake band sets.  A full overhaul kit is another £250 plus gaskets and a new filter plus new oil.

Any kind of pressure drop is likely to mean that the pressures holding the drive connections could fall too low and they start to slip.  Once that process begins then the overall box life may be limited and a full rebuild may be required in the not too distant future.

A lot will depend upon what else they find, and what fault codes gave been logged in the gearbox memory.

Fingers crossed that it's only the TC that's gone bad and you've caught it very early in the process...

No idea about the flushing process but I would imagine the gearbox rebuilders may well have the approved solution.

Richard

[X5MO]

UPDATE

Just to reiterate the car was driving as normal with very smooth power delivery and perfect gear changes in auto and manual modes.
6 weeks ago I noticed the fuel economy had dropped significantly and thought little of it but then I heard a VERY slight growling noise at set off which then developed through the low revs in 2nd and 3rd gears thereafter drive and sound as normal.

As above I have today spoken to the technician and viewed the gearbox with the metal sump panel removed, there aren't any particles in it visible or to the touch when feeling the fluid.
Tech said he will establish flow through the cooler and lines to make sure there isn't a blockage of any sort.
After which the whole thing will be put back together and topped up. No codes are present.
I will have to wait and see what happens when I get the car back and have put some miles on it in different environments or it just dies on me 

[kkx5]

Its more Transfer box issue not gearbox...

[X5Sport]

With discoloured gearbox oil giving all the indications of having got too hot?  Seems a little unusual for the transfer box to be the cause of that, they're separate from a lube viewpoint and it would have to get mighty hot.

1st to 3rd are all on the same gear set. Which might be why it's only on those three?  What makes you think transfer box out of interest?

When my TC failure occurred it made exactly the noise the OP describes.  A failed lock-up sprag clutch component in the TC caused all the trouble. Bit was bad luck that my oil pump got jammed by the swarf/broken parts and that was that.....

Richard

[kkx5]

Discolour its normal. Its never clean inside gearbox after 100k. 2,3 gear its when transfer ecu switches from 50:50 ratio to 40:60 hence noises.  I might be wrong. Its just second opinion.

[X5Sport]

Tnx.  Useful to know all the possibilities 

[X5MO]

I initially spoke to my local Indy and having checked for codes which weren't present he took it for a thorough test drive while I was sat with him.
He took the car on a hill and tested it in the lower gears and was very confident in his prognosis.
He has credentials too, BMW 18 years, IMI master technician and can sort anything you throw at him.
Local BMW/ mini dealers do send him work.
Anyway he is only human so I take on all your comments and appreciate the support and in that regard thank you Richard your posts have helped me

Regards
Mo

[X5Sport]

If there are no fault codes then take that as a good sign. 

Any kind of clutch/brake slip will show up in the memory so as they aren't there it points to things still being in tolerance. 

Richard

[X5MO]

Got the car back this evening, I drove it for the short journey home and it does change gears earlier and seems to be smoother than before but I will drive it for a few days and report back. The sound is less apparent but I'm afraid it's still there !!
I think I will have to get it into the Bmw Indy and get him to plug it in again and at least I can be sure it's not the TC. I don't feel disappointed as I did get confirmation that the TC was definitely worn so better safe than sorry.
So its cost 150 in fluid, 250 TC refurb, 220 labour for removal and re fit.

I wonder if its the diff ?? I did get that checked when I bought the car but will ask for it to be checked again.

[kkx5]

Pull fuse no 46, Transfer box.

[X5MO]

[quote="kkx5"]
Pull fuse no 46, Transfer box.
[/quote]

What will that do? Is that the engine bay fuse box ?

I've been reading a post which you guys have commented in regarding transfer box noise due to tyre wear difference between front and back axles.

I know Rears are  315/35 R20 but a month ago I put on Pirelli scorpion tyres 245 x 45 x 20 on the front alone

My car which has 20 " wheels, I think they are BMW light alloy wheel, V-spoke 168 and I think they are 10.5j x20 rims all round.

Could this be the cause of the noise ? Help !

[kkx5]

Fuse will disengage x-drive sysem and noise will be gone. You will find this in glove box. If the tyres You put are 245 not 275, that could be Your issue. Pirelli scoripons are winter tyres. Can not be mixed with summers. Seems like your indy has got a knowledge but not bmw x-drive knowledge and You waste Your money . Good mechanic will spot tyres issue and will start with transfer box fuse with no cost, not gearbox.  As i said before, your noises are down to transfer, wrong tyres or transfer need to be replaced. Pull the fuse, let us know...