You will NOT necessarily hear the valves hitting the pistons. IF they were hitting, chances are they would be bent out of the way by now.
I bent all 8 exhaust valves at the strip once... back before the car had a rev limiter. I was powershifting into 3rd at 6500rpm and the friction surface came off the clutch. The car was WFO @ 6500, then the clutch was depressed (still wide open), tranny put into 3rd (still wide open), then the clutch released.... only to find nothing there. After realizing what happened, I backed out of the throttle.... but not until the tach was at about a bazillion rpm!
I was trying to get the car into the 12's back then, and had a best of 13.10 that day. After fixing the clutch that night, I went back the next cold morning at 8:00 only to run a 13.25! It should have been in the 12's!!!!
Turns out, After checking the cranking compression and seeing it was down around 10-15psi on every cylinder, the car had eight -slightly- bent exhaust valves that needed replaced. A MSD 6AL (relatively new and out of my budget at the time) was purchased shortly there after.
Anyway... enough of that story. Point is, bent valves aren't always that obvious. If they were bent to the point that the car was running rough, the compression on those cylinders would be WAY down. Plus, I would think the bent valves would be most noticable at low rpm where the gases have more time to escape the cylinder through them. Do a compression check to verify the condition of the cylinders. If it checks out, you're going to have to go over the rest of the car a little more carefully.
Good Luck!
