Strange High Pitch noise seems to be coming from rear (4.8is) - any idea's???

[Lion Heart]

Hi guys

Recently picked up a 4.8is!!!

It has a couple of niggles which I need to iron out, which is not an issue as bought it quite cheaply and will be a cracker once done.

My first issue is a strange, high pitched permanent noise which seems to come from towards the rear of the car. Seems loudest around the rear wheel area.

I thought first of all maybe an exhaust leak, but if you do anything electrical then the pitch of the noise changes, so definitely electrical in nature.

I've taken a video (27 seconds). Have a look:

[url=http://"https://www.dropbox.com/s/b0ua362zw8...N2720.AVI?dl=0"]https://www.dropbox.com/s/b0ua362zw8...N2720.AVI?dl=0[/url]

In my video you will note I whistle at the start to mimic and help you hear the noise I am talking about - and as you will see when I push the rear window switch you can hear the pitch changing - nence pretty sure it must be electrical!

Surely they can't all make this noise!?!?!

Any help really appreciated as it's really annoying!!!

Thanks guys!

[X5Sport]

Could be the air compressor for the air Suspension or fuel pump.  Not much else back there that I can think of.

The air pump is in the spare wheel well inside the domed cover.

It should be silent, and should only run if the air bags need topping up so not all if the time when the ignition is on.

[Lion Heart]

Ah - making some sense, like it!

I'll open up the boot with it running and see if the noise is coming from there.

If not, how would I test to see if it is the fuel pump? Is that locatyed within the tank under the rear seats??

[X5Sport]

Not sure as all mine have been oil burners.

Look at Realoem (BMW Parts database) and use the last 7 of your VIN to pull up the right model.  It will tell you what is inside the tank, and what may be under the car.

[Horizon]

[quote="X5Sport"]
Not sure as all mine have been oil burners.

Look at Realoem (BMW Parts database) and use the last 7 of your VIN to pull up the right model.  It will tell you what is inside the tank, and what may be under the car.
[/quote]

This is where the fuel pump lives.
https://www.google.co.uk/url?sa=t&sourc ... 8Fb5jKE-qw
Usually he fuel pump primes / runs as soon as the car is unlocked.
Could be the air compressor, but that only runs every now and then. Is the noise constant, or when running, as soon as the car is woken up ???

[Lion Heart]

It's constant, as soon as you turn the ignition and does not stop even after a run, 100% uninterrupted.

So likely that rules out the air suspension pump I guess.

[X5Sport]

Unless you have a leak or failed pressure sensor which means the pump keeps running  :'(

[Lion Heart]

Yes, I guess so. The suspension does not leak overnight etc, i.e. it's at the same heght it was the day before when you come back to the car. I assume if there was a leak the level would drop?

[IanP]

Does the fuel pump have a suction filter? Could be choked.

[Lion Heart]

Good point. I guess maybe the pump could be overworked if the fuel filter is blocked??

[IanP]

RealOEM only shows one filter with pressure regulator. A blocked filter would certainly overwork the pump.

[Lion Heart]

I'm going to run some tests on this later and let you guys know. The main thing that bothers me is if it were the fuel pump under the rear seats, or maybe the air suspension compressor in the boot, then you would hear the nose inside the car, right?

I don't recall ever hearing it inside, but i'll check later for my own sanity...

[Lion Heart]

Hey guys!

Sorry for the delay but a mad couple of days with work etc.

Ok, update on this...

I listened very carefully again first in the boot for the air suspension compressor - zero noise there.

So listened from the rear side of the car next to rear wheel. Noted the noise is loudest under the car, and loudest on the right side (offside).

So next I listened inside around the rear seat area. Didn't hear too much, but there again the engine is pretty loud (4.8!)

So pulled the seat bench up and put my ear next to the fuel pump cover. I could hear the squealing through it for sure, but nowhere near as loud as it is outside the car (like 10x louder). But also I could hear what sounded like a slight clicking/grinding noise coming from there also. Quite faint, but definitely there.

It sounded like the ticking you get on the old manual kitchen timers but with a slight grind to it.

Someone mentioned the possibility of a ground loop/bad battery connection - butI didn't try the ground loop/check battery yet as a) the battery is a bugger to get to, and b) nothing else electrical is giving any issues at all, and c) I had limited time.

What do you think?

[Horizon]

Have you tried seeing if the noise alters taking the fuel filler cap off, if its fuel pump related you would expect the noise to be evident via the filler neck.
Just a case of elimination of possible causes of ignition powered components.
Good luck.

[Lion Heart]

Damn it - why didn't I think of that!!! Yes, makes sense. I'll try again later and pop the fuel cap off and have a good listen.

Will let you know...