I have a very subtle knocking noise from the engine compartment. Don't think that it is engine related as it does not speed up with increase in RPM. Generally can hear it with radio off only as it is that soft. I thought that it might be AC related. Naturally can't hear it when driving only at stoplight and it is very soft.

Running 91+ oct

2001 C320; 89xx on odo.

Any ideas?

 

regeneration valve