Have a pesky tapping/nailing sound coming from my 1981 240D. I know there are tons of threads already in existence about this, but after looking through them I am still stumped, curious to see if anybody has some wisdom to share.
Here's what has been done so far:
1) Valves adjusted
2) Oil changed
3) Vacuum pump checked. Was replaced by PO and ball bearing look almost brand new. Ran engine briefly without VP installed and noise persisted. Almost certain it is innocent
4) Alternator, water pump, power steering all checked for bearing play and anything else that could make a cyclic sound
5) Injector hard lines loosened one at a time looking for a change in the sound, no change heard
6) Inspected the rocker arms and valve springs for excessive play and/or broken spring
7) Run the engine with the valve cover off to look for obvious problems with the valve components. Did not notice anything, and oil flow was good. The only thing I noticed was white smoke coming from the timing belt compartment. Not sure if this is normal or not.
8) Measured the frequency of the tapping, found it to be ~6 taps per second, which seems to be valve speed rather than crank speed.
After doings the above and still not knowing the nature of the sound, I splurged and bought a mechanics stethoscope at HF.
Using the stethoscope I narrowed the noise down to the valve train or timing chain. Can't pinpoint a specific cylinder, the sound seems to be fairly uniform all across the valvetrain. Noise is not coming from VP or IP. Sound does not change when the engine warms up or when it is cold, and is proportional to RPM.
All signs seem to point to the noise coming from the valves, I just can't figure out why. If a valve had dropped or a spring had broken, it would be pretty obvious by inspection or when adjusting the valves right? Is there something else I should be looking at?
Another idea is the chain guide or tensioner or some other part of the timing chain is worn. Would that cause the 6 taps per second frequency though?
Maybe an intake manifold leak? Or multiple nailing injectors, which is why the injector line test didn't work?
Here's the link to a YouTube video of the noise, maybe that will help.
https://www.youtube.com/watch?v=ssPsRr6qgZA
Any ideas?