[83capriceclassic]

I recently had the ecm replaced in my caprice after the car mysteriously started backfiring one day while putting load on the engine. The shop also replaced a couple other sensors and the EST module.
The car runs so much better now, but the engine now makes a rattling noise going up hills on the highway. But this only starts occuring when the engine reaches operating temperature because on the drive to work there are 2 hills on the highway I take, it doesn't rattle on the first one but it always on the second one much farther down the road. I replaced the EGR valve and the vaccuum solenoid and they are working now as per the dianostic tests in the shop manual. I tried to check the timing but alas my timing mark is no longer readable so I really have no idea if the ignition timing is correct. I'm pretty sure the rattle is due to pre-ignition because it is reduced when using premium but it doesn't completely go away. I also tried Seafome and that didn't do anything either. Does anyone have any suggestions as to what I should check next or what might be the cause because I really can't afford to take this car back to the shop and have them charge me and arm and a leg and keep my car for 4 days trying to figure out the problem.