JimK wrote

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.

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...

Yeah, it sound more like there is something else loose inside and that's
why the books stop it, because it forces the largest side against what is
loose against that side. You wouldn't normally expect pressing on the
largest side would have any real effect on a cone coming to bits.