V6 gta turbo random knockimg/clunking sound

Hi on my 1990 I have a clunky knock ING sound that seems rotational it can be loud, slow, quiet, fast it generally happens whilst coasting in gear upto a junction but not always, it also only seems to occur after a fair few miles of driving sometimes it happens and sometimes it doesn't it's very irregular.

I've checked all the wheels and callipers, pads etc for movement and all seems good gear box mount has been suggested and I'm leaning towards wishbone rubbers/suspension rubbers but it doesn't happen if going over bumps, it only seems to occur while coasting in gear?

Hopefully someone can poo t me in the right direction...

TIA.

Which GTA model, the heatshields and engine bay layout might make a difference top what could be loose.
How much slack is there in the driveshafts? is the front gearbox mount, OK? (It's not too hard to see when jacked).
Are the lower wishbone mounts tight to the subframe and in good condition? (I had one come undone on my 610 which created interesting noises and handling).
Sadly, the GTA has a bunch of known faults/solutions, but this one isn't one of them. Speedo, now you are talking...