Downshift Clunk
 

I know this has been covered before, but mine occurs when accelerating.

If I am going between (roughly 45, to 55), and I floor the gas, downshift to second, I sometimes hear a clunk. It only happens when cruising about 50 when downshifting to second gear. Any ideas?

722.4 on C280.

Is the noise coming from the gearbox or the back axle?

How many miles on the C?
Could be many things, first check fluid level, and trans filter.
Second inspect all the components of the driveshaft.

Could also be wear (free play) in the rear differential.

I have 121k miles.
Trans has been serviced by dealer at 20k intervals or less. I am due for a 30k mile service pretty soon.

Lower control arm bushings have been replaced, as has the front flex disc.

The noise sounds like it's coming from below me or in the engine bay... pretty sure it's not the rear.

Thanks.

Can you replicate the clunking by holding the car in a lower gear and rapidly accelerating? Try to replicate the engine movement that would occur with a downshift and sudden increase in revs - you might find that it's actually something like an AC hose or similar that knocks on something when the engine suddenly revs out.

Cheers
Matt.

It doesn't happen on any other occasion, except when going into 2nd @ high RPMs. I just had the trans serviced a few k miles ago, and the fluid level looks accurate. OTherwise, it shifts wonderfully.