/ If turning the speaker on its side tends to remove the nasty noises I'd say the speaker suspension has sagged - quite common. What happens then is the voice coil scrapes on a pole piece, making the noise. If you can get inside it, invert the speaker.

If you can get at the speaker itself, try pushing the cone gently with your fingers applying pressure all over. You can usually feel it scraping if this is the problem./q

Mmm ta. The difference seems to be when I lean on the (largest) side. I currently have a pile of cook books on there and it's belting away quite happily. Does that fit with the voice coil thingy?

It's as if somethings resonating but can be damped by a pile of cookbooks/me leaning on it...

Jim K