Funny loud noise

My '75 Kombi is making a very strange noise in the front. If you put the brakes on at about 20 k/h or so, when the steering is turned a little to one side, it makes a loud shuddering sound just like as if you've run over one of those cattle grids you find on farms. Only thing is, there's no cattle grid ! I thought it was the brake discs but they look fine. The calipers aren't loose or anything. The top ball joint rubber covers look pretty crook, but I jacked it up and can't feel any looseness. Any ideas ? It been happening for a month or so, at first just occasionally, but now it's getting more often. It only happens at slow speed.

loose pad???

I would pull the pads and take a look

May sounds a bit silly but:
is the wheel on tight?

I don't know where you are (that'd be because you haven't filled in your profile) but down here you can get a suspension/shocks test done by Pedders for beer money, which will certainly highlight any major component problems in your Kombi.

Just looked it up it's a "28-point suspension safety check" for $14. You don't usually get much for $14 these days...but that's another story.

hth