Valve lash does make a big difference, so check that out as mentioned previously. Also, the cast intake and exhaust manifolds are quiter than the thinner headers that are available. I'm told that for most non racing engines, the stock manifolds are better than a lot of headers anyway.

You can also check local salvage yards for sound deadning material out of luxury cars (or buy new stuff...dynamat is what the really freaky car audio people use for sound deadning). As far up the firewall down to the rear of the car is where you want to apply it for best results. Add some to the inside of the doors, and finally add the proper deadning (I'm I spelling that right?) to the inside of the hood.

Tremmelle