My valves are rattling when I gas it

Not sure where it cut off but the oil and plugs were changed at 998500 and the valves rattled so the mechanic put Seafoam in the oil to quiet them and it worked for a couple days then I noticed a flutter in what sounded like the bottom end, sort of like the old heat exchangers used to make, like a faint knock. The mechanic couldnt diagnose and said wait till it gets louder. Meanwhile the trans breaks and needs rebuilding. Aamco assured me thats what the knock was but $2400 later and its still there and a little louder. My mechanic says take it to Cadillac but that will cost me an arm and a leg and still may not know without a tear down of the motor. The noise is only there on acceeleration

I'd like to add that it doesn't start to knock until it's warmed up, about 2 miles into it.

I think they put 10-30. 5-30 is the recommended oil I believe. I was going to use the "High Mileage" oil in the recommended weight and the lucas oil treatment. Does that sound right or should I be using a different oil? I've always used Castrol in my cars but I've gotten older and lazier and now pay someone to do it. Appaerntly a bad move on my part. Do you think the Seafoam had something to do with it? Ran quiet and smooth before the service. It even runs a little rough now like there's a miss or a plug not firing but the onboard monitoring system says "All monitored systems ok".

5-30