Just a thought,
A few people have been saying that there car has a vibration at 4000 rpm, now a thread has just been posted about the plastic engine cover, could this be the root of all evil.
The cover is the thin plastic one that sits on the engine top and covers the Plenum chamber, as the engine hits 4000rpm maybe this resonates ?
Why do you think ppl ?
Plastic against metal would sound like this !