CLK320 W209 pulsating rattling noise from engine after cold start (with video)

[tannedpotato]

Hello guys,

I recently noticed my 05 plate CLK320 making a weird metallic rattling noise under the bonnet, which "pulsates" rhythmically in 2-3 seconds intervals. The noise will come up without fail after a cold start (having sat for several hours), but will go away after 15-20 minutes of engine running regardless of speed/rpm (I tried letting it idle for 20 minutes and the noise went away).

None of the garages I took the car to could point out the culprit, mostly because by the time I got to the garages the noise has already stopped...

A Mercedes specialist said based on my description it might be the timing chain, seeing my car doesn't appear to have had one replaced (96k miles on the clock). Although this noise sounds slightly different than the typical timing chain noises on YouTube to my untrained ears.

Another indie garage said it might be the tensioner, although they were again unsure because they couldn't hear the noise in person.

I managed to record a short video of the noise, taken within 1 minute of cold starting the engine. Can anyone help identify it please?

Thanks and cheers!

 

[Doug1234]

Sounds like a bad idler or tensioner wheel , I say this because I have two idlers and the tensioner wheel to fit to my engine and although I do not get quite the same noise as your whirring mine goes silent after about two minutes.
Mine clicks loudly until the bearing warms up a bit and then its happy. I replaced the alternator clutch pulley and realised the idler wheels just spin forever when given a flick to get them going when they should just stop after about 3/4’s of a turn.
Also find better garages, if it is an idler or tensioner wheel a competent garage could fault find it with a stethoscope within minutes.

 

[tannedpotato]

Sounds like a bad idler or tensioner wheel , I say this because I have two idlers and the tensioner wheel to fit to my engine and although I do not get quite the same noise as your whirring mine goes silent after about two minutes.
Mine clicks loudly until the bearing warms up a bit and then its happy. I replaced the alternator clutch pulley and realised the idler wheels just spin forever when given a flick to get them going when they should just stop after about 3/4’s of a turn.
Also find better garages, if it is an idler or tensioner wheel a competent garage could fault find it with a stethoscope within minutes.

Thank you Doug! It does sound like one of the rotating bearings and would make sense with the 'grrrrrr' noise.

I'll cut the garages I visited some slack though, as I have yet to leave the car with them overnight so none of them actually heard the noise in person, let alone examining with a stethoscope. I've arranged to leave it with the Merc specialist later next month and hopefully they'll be able to sort it.

I'm just glad it's not likely the dreaded timing chain! Although with my mileage I suspect I'll need it done sooner rather than later anyway.

 

[alexanderfoti]

It seems too high a frequency to be a chain noise.

I would remove the belt, leave it overnight and start it up when cold (for 2-3 mins tops).

Sounds like it might be the alternator

 

[rifiki]

If no documentary evidence change the idler and tension pulley. Check smooth operation of the rest, turn by hand.
The tension pulley let go on my 05 CLK (M272) with no warning. The shredded belt wrapped it's self round the crank shaft between block and pulley causing more problems. It let go at around 85K miles

 

[mioba]

Sounds like a pully. The oil in the bearing gets used marginally over the years, and when cold doesnt do its job. once warm its spread out and can lubricate (so noise when warm).

Get the serpentine belt off, check all pulleys for noise and play and replace accordingly.

Change the belt while there and the tensioner.

I change my belts every 10 years as a precautionary measure.

The belt and the pulleys do a hell of alot of work.

 

[rifiki]

It should be a service item.

 

[DREAMER NO2]

Also check the crank pulley and the bolt securing it .Wont be the first to come lose .

 

[tannedpotato]

Thanks everyone for the input, much appreciated!

After some diagnostics it turns out it was indeed the alternator that made the noise. I did not get to find out what exactly went wrong with it though, since I opted to replace the entire thing with a remanufactured unit which was overall easier.

All the pulleys were actually in good working order, although since the serpentine belt was being replaced, I chose to replace the idler pulley, tensioner assembly and crankshaft pulley anyway as preventive maintenance.

Finally the car doesn't make an absolute racket every time it starts from cold!