Knocking after period of driving

Evening all

Got a bit of a strange one which I need suggestions to progress with. Firstly - I think I need to replace the rear wheel bearings as there is a bearing noise coming from the rear - its on the to do list. What concerns me more is the very loud knocking noise I am getting. Imagine if the exhaust was hitting the chassis at the rear sort of sound. The strange part (to me anyway) is that the noise only comes after a period of 20-30 mins of driving and is very hit and miss. Once it starts its usually 5/6ish "knocks" then nothing and continues sporadically for the rest of the journey - not linked to accelerating or turning. In fact it happens frequently driving in a straight line on the motorway at 70? Could it be linked to bearing? Any ideas appreciated.

Regards

Freelander 1 - 51 Reg 1.8 GS 5 dr.

Wheel bearing would normally be more of a drone.
Other possibilities are exhaust and rear diff bushes.........if the rear diff bushes have been changed recently and the noise started after this....then there is also the possibility of it being rear diff drive shaft carrier bearings.

Shocker? Broken spring? Droplinks?

Cracked Chassis?

Common problem on earlier Freelanders

Loud repetitive bangs (like someone whacking metal with a hammer) sound like gears slipping in the IRD or diff - I wouldn't have thought it would do it for long before complete failure though.

Whats your tyre wear/pressures like?