Your 1991 Mitsubishi 3000 GT sounds like a sewing machine you had the oil changed and it still sound the same the car has 149000 miles What could it be?

if this sound is coming from the "top" of the engine it may be cause by the lifters. the easy way to get rid of this with out mechanical help would be getting a thicker oil. and also there is a liquid , "engine lucas" which it helps your oil lubricate your engine components better. try that.