One twist in this is the two vacuum pumps are not identical. On the right engine (the one "buzzing") it's an Airborne model 21500. Looks similar in design to a Tempest or Rapco. On the left engine (not making noise) it's a Sigma Tek model 1U128B. According to the Comanche IPC the correct part is Piper Part 481 692 which is a Dry Vac Pump #200CC for my aircraft. I am still researching this but I believe the Airborne on the right engine is the original model (and was overhauled with the engine about 727 hours ago) and the Sigma Tek on the left engine is a PMA replacement.
The 2nd paragraph above is simply to say that the two models are different and perhaps that is why one makes noise and the other does not but my instinct is that is overly optimistic thinking... I am concerned that even though the vac pressure from both pumps is normal, the noise coming from the right engine Airborne pump is a leading indicator that a failure of the pump will soon occur and / or it will soon give up the ghost. If so I'd rather deal with that while the cowls are already off as de-cowling and re-cowling these engines is not my favorite activity
I spoke to a local shop that deals with a lot of older aircraft and they were not sure from what I described. Thought I'd post here for some feedback.
Thanks in advance.
- Charles
