bad99transam

i have a 94 lt1 with 190000 miles on it. i just bought 2 months ago it runs really strong and better then other lt1s i have ran in to, but recently i have been hearing alot of valve rattle in 5th and 6th gears. is this normal or is it a sign of some parts ready to fail. the car has strong oil pressure and just did a full tune up. msd opti, coil ,wires , and plugs, fuel filter, oil change. any ideas?

mods to the car are, flowmaster catback,k&n c/a/i, trick flow elbow,bbk 58mm tb,mizier elec water pump, 3.73 gears and a short trow shifter, and a new clutch. the last time i dynoed it it put 271/311 to the wheel with a bad cluch.