I've got a B&D CMM 1200 (2009) cordless electric mower. This season it has developed a low rumbling noise, clearly some sort of vibration that is amplified by the whole mower body. It comes and goes, unpredictably, sometimes affected by moving/turning/tilting the mower and other times just coming and going even as the mower is held still over concrete driveway. I opened it up and ran it with the body top removed; no difference. I didn't find anything loose (I had replaced the battery myself so I was concerned that something may have gotten loose) but lightly tightened the three bolts holding the motor housing on to the mower body; no difference. Running it again it looked to me as though the motor shaft seemed to wobble/oscillate slightly and even spark a tiny bit and that seemed to coincide with the noise, but it also looks like the brushes or whatever they are called seemed designed to have a little give so I don't know if some wobble is normal. I've had the mower for five years, and have had almost no problems. Last year the mower's performance suffered and it seemed a little noisier but the B&D service guys said that was due to an improperly balanced/sharpened blade; balancing the blade fixed the problem. This is a different louder noise that comes and goes, and doesn't seem to affect performance. Anyone know/suspect what is going on?