i have a 1990 chevy 3/4ton with a 305 and my truck has never made a knokcing noise before but once i got my exhaust donuts changed everytime i slow down or stop all i hear is knocking. you can hear it slightly when it is in park but you hear it the most when ytoure in gear. any help?

 

need to check the oil pressure ,you may have a bearing issue if it is up to level and 20-30 psi .may be main bearing's