Farish Cl 66 clunk

[airfix27ra]

One of my Cl66 has developed a clunk when travelling in one direction only, ie when the end is travelling forward, it clunks,
when the same end is at the back, no cluck. One can see the cab actually lifting,(about 1/16) when clunking. This occurs roughly every
8 - 12 inches.
I have taken the bogies off and had a look, can't see any obvious problems. The gear teeth look ok and run smoothly
enough, no foreign bodies appear to be present. Very baffled. Any ideas?