M104 fast rattle/clicking under high load

[tcap]

My e280 s124 generally runs very smoothly. I've put 500 miles on it since I first noticed this issue, without incident.

However, there's a peculiar noise that emerges only above 4k RPM and at full (or nearly full) throttle. It's a rapid clicking or maybe rattling sound. It sounds like it could be a slipping ratchet or loose bolt (but I can't find anything superficially loose in the engine bay). I guess it's also possible it could be coming from the transmission (4spd auto) - it's hard to tell.

The sound doesn't emerge if the engine is revved up to high RPM in neutral, or at full throttle at lower RPM.

Any ideas?

 

[star]

Pinking or pre ignition? Try some high octane fuel???

 

[tcap]

Having just looked it up, yeah could be - I've been using 95 which I had read was fine. Will try 98.

 

[star]

A couple of tanks should make a difference if it is that.

 

[LostKiwi]

Has it recently had new spark plugs? If so are they the correct ones for the car?

 

[tcap]

I gave it 3/4 of a tank of super and still got the noise today, albeit maybe slightly fainter(?), but I'll wait until it's had a couple of tanks as Star suggests.

Then yeah I think next on the list would be spark plugs. It hasn't had new ones while I've owned it.

 

[tcap]

So I'm about one third through the 2nd tank of super unleaded now, and although it's faint, I still get this noise at high load.

Any spark plug recommendations?

 

[LostKiwi]

How are your exhaust manifold bolts and gaskets? With engine running see if you can feel any gases around joints and gaskets.

 

[tcap]

I can take a look - the car had a new exhaust not too long ago. Can you elaborate on how that might lead to the noise?

 

[LostKiwi]

I can take a look - the car had a new exhaust not too long ago. Can you elaborate on how that might lead to the noise?

You gave six branches coming from the engine into the exhaust. If one is leaking you'll get a pulse of exhaust noise every second revolution of the engine. If it's a minor leak it may not be audible but as engine load increases the gas in the manifold increases as does the pressure in the exhaust. That can manifest as a ticking sound under load.

 

[tcap]

Makes sense, thanks!