I have a 1991 Volvo 240 4cyl with 218xxx+ miles on it. A couple months ago it started making a knocking sound that I thought was the driveshaft hitting. I stopped driving it and figured it may have been the u-joints because I had recently replaced the motor and transmission mounts. They were bad, so I thought that could have damaged the u-joints. I replaced all 3 and the sound went away for a couple months until I backed up an incline and the exact same noise started again. I drove it for awhile and it went away. Then it came back again when I backed up a small incline, which seems to cause it initially whenever I do this. The noise only happens when switching between driving and coasting at low speeds, like below 25mph. When driving any faster than this, the noise is nonexistent. No signs of anything obvious hitting underneath.