I have a 87 chevy caprice with 83,000 miles on it just wanted to know...?

i wanted to know if its ok to do a engine flush on it? i mean i did it already it had alot of gunky build up in it...also i got a mobil high mileage oil change...should this be all ok? i have some chattering noise comming from the engine i dunno what it can be? i thought doing a flush and adding high mileage oil will quiet it down and adding a oil additive but it didnt...people have said it could be many things lifters valves? i dunno what it is...