Rumbling rear and scuffing from brakes

I've been noticing a pulsing rumbling noise from the rear of the car (1.4 Comfort Estate, 2003 reg). Not always there but recently joined by a scuffing noise when I brake which is clearly related to road speed. This scuffing isn't always there and sometimes happens under heavy braking and other times light so really inconsistent.

I've had it checked by a garage and it's just been through the MOT.

Today I've jacked it up to look for myself. I can find no bearing play or rumble. Both rear discs are clean and straight, pads good and brakes come on as they should when the pedal is pressed and release cleanly.

One of the plastic shrouds on the rear shocks had come adrift so I've re-attached it with tape to the bump stop.

What I did find though was that both rear tyres have visible flat spots - the left side has palm sized patches of low tread but the right just has some tread blocks that look lower than those next to them. Does anyone know what could cause that?

(I recently swapped a few of the tyres around to see if I could fix the rumbling so I can't say that they originally got like this on the rear of the car. The car came on a mix of different tyres.)

I'm going to replace two tyres now but don't want to wreck two new ones. Any advice would be greatly appreciated.

Thanks