Strange Engine Sounds: A Case of a 3.4
 

Okay, here goes my attempt at getting this solved via internet. Anyone with suggestions, feel free to chime in:

-currently @ 66,000 miles


Before purchasing a 2000 996, I drove two seperate base model Boxsters. I had passed on both mainly because they exhibited a loud and obnoxious buzzing noise from the engine/exhaust area while accelerating/down shifting. Lucky for me as I went on to find a wonderful (but quirky) 996.

Sound #1 is, I am now experiencing a similar noise to the one described above, albeit much quieter and under limited circumstances. It occurs under moderate-to-heavy throttle load when at lower RPMs (1500-2500). It is almost a diesel-esque sound; a metallic clicking noise. When revved at a standstill, the noise cannot be heard!

Sound #2: Upon starting the motor, both when hot and cold, it sometimes emits a very "dry" and "starved" grinding sound while letting out a noise that I can only describe as a whirling/fluttering feathering sensation as idle is reached.

I've tried searching this site but no one has described anything of the kind. My mechanic replaced the idle sensor while performing the 60k service. Neither has helped.

So what happened now?

#1: could be a lot of things including just a loose baffle in a muffler or something loose on the car somewhere -- seems to me weird resonate buzzes occur on some Porsches if and when things haven't been checked and come a bit loose over time.
could be the engine, though: if there were a slight problem with the ignition timing, if the octane of the gas was too low, then you might be hearing knocking (pre-ignition). IOt it is occurring it would definately occur at low RPMs under heavy load (it might be occuring at higher RPM too and you not hearing it), and it would not occur when you rev it up just sitting still (you aren't loading the engine, just reving in then).
Not sure but it it was me I'd check. Try finding and running some real high octane fuel, or dropping on some good quality octane enhancer: if the noise goes away then you have found the problem. If any doubt about the engine and tuning, at 35K, I'd change plugs, wires, and do the full ignition and clean the injectors things -- I know its early but if the car had hard service and if the fuel you are using is marginal it might make a difference

#2:this could be quite a few things, including a starter solenoid that is sticking or a bad tune that makes the car smooth up to idle on starting very slowly, etc., or some oil starvation problem, or nothing at all. Is your mechanic worried? does he/she have experience with Porsches enough to know?