A loose spark plug can cause chirping or noise and although it would seem to be tied to RPM if it doesn't chirp every single time it could seem pretty random. Would be pretty easy to check that all of them are torqued down properly. My son's Miata had a weird noise recently that we couldn't figure out and then when he was driving it blew out a spark plug... Reinstalled it and torqued it properly once the car had cooled down and no more weird noise...
I will second the spark plug theory. Mine never made noise on idle though. Only when I put a load on the engine did I get a noise. Like about every time I took off in first gear from a stop. I would get some funny looks from bystanders
1998 Porsche 986
Hybrid Mode
