Squeak and tapping noise from valve cover during idle

[bryankberry]

I've got a 99 Boxster with 81k mi. The other day I started hearing a squeaking sound like a bearing going out while the car was idling and tapping sound, that went away during revs. I checked the idler pulleys and the noise wasn't coming from there. I got under the car and found that the noise was coming from the driver's side valve cover. Any ideas as to what could be the issue?

[911monty]

Check Spark plugs.

[BoxsterSteve]

+1
One or more plugs on that bank are loose and need torquing.
Check 'em all while you're down there.

[bryankberry]

Hands are still greasy but I wanted to jump on and thank both of you (BoxsterSteve and 911Monty) very, very much!!!! That was the solution! When you both said check the spark plugs I thought, "that's an odd solution to what sounds like a major problem". But sure enough, the middle plug was loose. Tightened it down, started it up and noise was gone. If you both lived in Houston I would buy you a steak dinner. Thanks again!