Ok guys, I'm a chick, but I know a little about cars so please help!   I have a 1970 Chevelle with a 350 crate motor and a 700R4 transmission.  I just had a governor gear (not sure what exactly its called) replaced in the transmission about a month ago.  All I know is it tells the car when to shift.  I know its unlikely that would have anything to do with it, but it reminded me of when you try to take off in high gear in a manual shift transmission, its sluggish and it rattles.  Anyway, since then, when the cars in overdrive and you get on it, the valves tap really bad.  It only does it when its in over drive though.  You can drive it all day in low gear and never hear them until you put it in overdrive.  So, I guess my question is this:  Could the replacement of the gear in the transmission have something to do with this?  Also, its been a while since I've messed with valves, at least 10 years, and sort of remember how to adjust them, but I don't want to over tighten because I know it could break a push rod.  LOL, and I forgot to mention that I live by myself and only have the standard tool kit like sockets, wrenches, etc.  Nothing fancy I should say.  So, the other question would be: what's the best way to go about adjusting the valves? Any help you can give me would be greatly appreciated!!    

Sounds like your engine might be detonating(pinging) when you step on it in high gear.  Did your transmission down shift easier before the gear was replaced?  The rattle might just be a symptom of your transmission not downshifting as early as it used to or that the torque convertor isn't unlocking when you load the engine. I would take it back to whoever worked on your transmission and have them check it out.