I had similar problems on my bx25d, unsettling rhythmic noises, jerking, etc. Thanks to good advice on tbn, I detached and checked the u joints on the drive shaft, as suggested above. The front one was falling apart. The back one was fine. Apparently this is typical. Disassembly and replacement were easy. Found the parts diagram on Messick's. The parts arrived in a few days from them, for under $100. Reassembly took less than an hour. Anybody can do it!