Rattle noise

[rajinder_1]

Hi Guys,
DOnt know where this rattle is coming from... i only hear in quiet small built up streets. The car is 2001 sl320 M112 engine.
It seems to happen at the low rev range as well. Thought that it may be the cat but have had car on ramp and tapped the cats to see if i can hear the rattle but nothing.....
any ideas as it really bugs me when i drop the window and hear it?????

Raj

 

[Birdman]

Maybe the exhaust system is not correctly hung and is tapping against the underbody. Difficult to know without hearing it. Can you put up a sound file...

 

[rajinder_1]

the thing is it isnt constant, only in a small built up street i can hear it... and its only when i accelarate..... i try and avoid that road now.....lol
but i was on a us site and they said it may be the bracket, initially i thought that and checked it was loose but it has been tightened and it still does it, the guy in the us also said it still happens but it is the bracket regardless.

raj

 

[Birdman]

the thing is it isnt constant, only in a small built up street i can hear it... and its only when i accelarate..... i try and avoid that road now.....lol
but i was on a us site and they said it may be the bracket, initially i thought that and checked it was loose but it has been tightened and it still does it, the guy in the us also said it still happens but it is the bracket regardless. raj

Perhaps the whole system aft of the downpipes needs rehanging. Rather than tightening the brackets they should all be slackened off and retightened only after the system has been correctly repositioned.

Otherwise, it could be the engine pinking, if it's quiet and happens only under acceleration. Unlikely 'though, as it makes a very distinctive light tinkling sound which comes from under the bonnet rather than under the car.

 

[rajinder_1]

what is pinking??? as the noise is a sort of tinny sound in low gear on accelaration

 

[grober]

pre-ignition/pinking

Its PRE-IGNITION where the petrol/air mix on the compression stroke ignites prematurely before the piston has reached the top of its stroke. Can be caused by many things chief of which are engine temperature,ignition timing, compression ratio and petrol octane rating. Modern engines have "knock sensors" AKA "pinking detectors" mounted in the engine block to counteract this and when the engine ECU detects pinking it alters the ignition timing to eliminate it. It can only compensate so far and sometimes cant eliminate it altogether. I would try filling the tank with some of the very high octane fuels now available and see if that makes any difference. Get the fuel tank almost empty of old fuel before filling up. Also check that your engine temperature is normal.

 

[rajinder_1]

I dont think that it is that tem,p and stuff is all normal, have been using super unleaded for a couple of weeks aswell

 

[Birdman]

what is pinking??? as the noise is a sort of tinny sound in low gear on accelaration

Pinking - pre-ignition - usually occurs when the engine is operating under heavy load conditions eg accelerating briskly from low-ish revs. It normally fades as revs rise.

Pinking causes tinkling and in even knocking noises from the engine, together with power loss. Potentially it will damage the top end. It's an unlikely culprit here because your ECU should back off the timing to compensate for engine load and fuel rating but if it isn't doing this it don't much matter what fuel you're using.

A small gas leak from a poorly sealed union can also make tinny or squeaky noises, especially at high load/low revs.

It could even be you have a family of singing rodents dancing on your hot exhausts, so lots to think about!