This may sound crazy but I worked as a tech for dodge dealers thru the 70's and they had a problem with the 360's, when if the motor backfired it could pop the seat off the EGR valve .Dont know if your car still has one on it but it makes that type of noise. EZ to ck...just take off the valve and look at it....the little plunger should be seated in the valve body... Just a thought