I recently bought a '92 740 with only 94,000 mi. I took it on the road about a month later, and noticed it began to backfire at high speeds (over 65) after about 2 hours on the road. These are not occasionaly loud pops, but rather the sound a 1960s hot rod makes when you take your foot off the gas, a rather steady rumbling. At first, it did this only with my foot off the gas (slowing down), but on the way home it started to do this in running position (70 mph). This has remained intermittent. Also, starting after the first trip, it backfires at very low speeds (such as backing out of the garage) --- this sounds like a heavy-duty rattle in the rear muffler, but when I put my head down to listen, it sounds more like the popping of backfiring. Again, this is a steady sound, not an occasional misfire. But it stops after 5 mph and above. The other thing, it seems to be worse when the engine is cold, in the morning or after sitting at the train station all day, but it is beginning to occur more frequently.

Does anyone have an idea about this? Could it be an exhaust manifold problem, bad timing, or poor valve action? Could the timing control mechanism attached to the coil need replacement?