Unusual noise when braking

[rincewind]

Hi all,

Firstly, it's worth saying that the car tracks perfectly straight, no wobbles or vibration.  However, under anything more than light braking I can feel/hear a noise that sounds like the car is going over tram lines.  It's not at all loud but certainly it's there, like a rather muted thump thump thump thump (frequency about 3 per second).   I've not come across this before so I'm a little perplexed as to what it could be.

 

Any ideas?

 

Cheers

Paul