I have 2 f7 a and b units that I purchased several years ago. from the get go when I ran them there was a clicking noise as they run around the track. I tried to figure it out and narrowed it down to the gears in the trucks. I sent them into bauchman and for the first few times run fine.  the noise has returned and is very loud. when I switch directions it changes pitch or dulls down.  does anyone have a cure?