Our 2004 Altima S had the subframe replaced due to rusting issues. The new/used subframe looked very good and came with a warranty. My frame mechanic said all the rubber attachment points (forget their real name) were in good shape. Within days of picking up the car we would randomly (once or twice a week) hear a tremendous knock on starting the engine with my feet flat on the floor and hand not on the shift lever. It sounds like a heavy sledgehammer hits the car and it reverberates all over; mostly it sounds like it's coming from the floorboards but sometimes it sounds like it comes from the roof (maybe due to parking in the garage). And now the transmission seems to give a little soft clunk when putting it into drive, which it didn't used to do (I'll check the fluid). My assumption has been that one of the frame mounts was not good and the engine/transmission was slipping up onto the edge of the subframe and then dropping into position when the engine starts. I took it to the frame guy and he put it up in the air with me in it and had me start it 20 times, but there were no unusual knocks or anything; but the heavy knock continued very randomly. We now have not heard it for a month (but once heard it on starting 3 times in a row). Could the transmission give such a tremendous knock when it was in park when the car was started? All suggestions of where to look now are welcome.

Did anyone examine the subframe bushings along with the mounts? The subframe is itself rubber-mounted, four big bushings at the corners. They're cast into tubes in the subframe and held captive with big fender washers, but if one fails by coming loose from the frame tube, it will let that whole corner of the subframe rock with the engine. The other possibility that comes to mind is a loose flywheel bolt. Those will often allow the flywheel to flex while the ring gear is being driven and then return to position with a whack when the engine catches.

Yes, bushing was the word I was searching for. My frame mechanic looked at the bushings and determined they were in good shape, but that was the first thing I thought of, or an engine/transmission mount. Your comment about a flywheel bolt is something I never would have thought of. Today I spent a little time putting it into drive and then reverse and there is something that is taking place that the car normally does not do. There is a looseness to the changing of gears when stopped; when driven, however, it shifts properly with no unusual sounds or behavior at all. Passing gear is right there working correctly. But when I stop and it returns to idle, there is actually a roughness to the idle that has not been there and which diminishes as it is revved up but does not disappear completely but seems to be masked by the increased rpms. It makes me think of a vacuum leak but I can't imagine how the thunderous knock (only one loud knock at a time, never two or more) could be related. The transmission fluid is at the correct level and looks like the right color and it doesn't smell burnt. I watched the engine as I started and stopped it and there did not seem to be any unusual movement just a little vibration on start. The check engine light is not on but I'll put my code reader on it anyway and see what it says.