Engine makes a rapping noise at idle and under throttle.

Hello. I have a 2001 Olds Alero with a 2.4L engine that recently started making a "rapping" noise at idle, under throttle, but most noticeably when easing off the throttle and the RPMs start to drop back down. The noise is coming from the passenger side of the engine, in the area of the timing chain cover and the valve cover. Thinking it was a noisy lifter, I drained 1 qt of oil and added 1 qt of Marvel mystery oil to quiet it down without improvment. I changed the oil and on the suggestion of a local mechanic, drained 1 qt and added 1 qt of Lucas Oil stabilizer without improvement. The noise is very low at idle and can barely be heard above the general sounds of the engine. The frequency of the rapping increases with rpms and can be heard easily while accelerating. However, it's most noticeable when the RPMs start to come back down or just as the transmission is shifting. A local mechanic listened to the noise and instantly diagnosed it as a valve problem but had no immediate suggestions aside from the Lucas oil stabilizer.
The car was idling rough and showed some hesitation when accelerating but the problem was resolved after changing the plugs and coils.. Mileage is good, no obvious misfires and everything else seems to be fine. Any assistance would be helpful. The car has approximately 103k miles and isn't leaking any fluids that I can tell.



Thanks!

Brian, North of Boston