Noisy "valve train"/ ticking/knock
Just thought I would share something interesting that I stumbled upon,
My E36 M3 had what most would presume to be a noisy lifter or valve train tick along with what sounded like a mechanical knock around cylinder #3/4, the vacuum gauge had a solid reading so I decided to do a compression test and CLT. While removing the coils and plugs I discovered that a few of the plugs were loose (barely hand tight) inside they're bores. I continued with the tests and they all passed. Upon reassembly I (being anal and thorough) torqued the spark plugs down to they're specifications and presto bango problem gone. The spark plugs not being the correct torque specs created the noise as the compressed air was forced against and them and over time as they began to loosen the noise continued to get worse until it went from the ticking to more of a knocking.
So for those of you running into a similar issue, don't forget the basics or the obvious- or just get lucky like I did!