it's worse than I thought

Many years ago, my first slant was rebuilt by a college auto shop - I was in another phase - tune-up, I think, Anyway, came time to drop it back my little Dart GT and it fired right up. And, for the whole time and many, many thousands of miles, it had what sounded like the same thing - wrist pin knock - only at idle, and kind of off and on. When I sold it to buy my 68 convertible GT (with another slant), it was still running fine. But with the money you paid, your rebuilder should darn well help you out!