I was having the same problem after driving to canada last weekend. All the way there under any kind of acceleration in any gear I would get a rattling/pinging noise. I had that noise before when I had a cracked manifold, but the other noise that accompanied it wasn't there this weekend. When I got home I changed the oil (it was due for it) and decided to add a bottle of valvoline Max life to the crankcase, since I have 96,600 miles now. Its supposed to help the seals expand and quiet valve noise.

All I know is that now, 1 day later, I get no pinging at all, even under hard acceleration, so it may have solved the problem. It was also idling rough, and starting in first gear was really hard. I had to let the clutch out super slow to get it to go without jerking like crazy. I still plan on taking it in and having them go over everything before I hit 100,000, but the clutch problem has disappeared since the addition of the Max Life also (I know its probably unrelated). Its only like $4.00, maybe worth a shot.

-Tim