Micra - vibrating exhaust.

Hi

1990 Nissan Micra automatic. I successfully replaced the engine with a good second hand engine. Trouble is the exhaust is vibrating and knocking now. When standing there is no noticeable or unusual vibration in the engine or exhaust, even when I rev up. However, when driving it's different. When the engine is under load, ie accelerating all is fine but when I take my foot off the throttle the exhaust begins to vibrate. I have checked the rubbers and they seem OK. Before I changed the engine there was no problem. It's an old car and I don't want to throw much more money at it trying to fix it. On the other hand I've spent a lot of time on it changing the engine and don't want to give up on it . Any ideas? TIA for any input. Paul.