As I said in the previous post one of the engines I installed a TCS M4 decoder in turned out noisy. Some kind of mechanical noise. So I sat out to fix that.
The first thing I saw when I disassembled the loco was a tiny piece of wire insulation on top of the gears in one of the trucks. Aha! Simple as that. So I removed that (strange I did not see it when the loco was assembled since it was shiny red). Back to the track, but the noise was still there.
To make a long story short, the noise turned out to come from vibrations in the draft gear, spreading to the frame. Don't ask me what caused that. I don't have a clue. In the end I managed to dampen that vibration by applying some grease around the gear fittings. The engine was assembled again and ran much more silent. There was still some noise when going in reverse, but I decided I could live with that.
I then played around with the two newly decoder equipped locos, MU:ing them, programmng light effects etc. Then all of a sudden, the formerly noisy engine started to jump ahead by its own (throttle down to zero) with flickering lights! The decoder seemed to give off short and regular bursts of power to the motor. If the engine was removed from the track and then put back again it first sat silent but after a while started to "jump" again. I tried a factory reset and other tricks but nothing helped. In the end I took the decoder out (it still behaved in that way when not attached to anything but the motor and track power) and sent it off to TCS as a warranty issue. They promise a "no questions asked" guarantee scheme so I hope to get a new one back at their expense. Sigh!