Bad brake calliper piston
Just a little info in case anyone else is having the same issue. Have a 2009 Mini Cooper Clubman (diesel engine) with 130,000km (80,000miles). For the last two weeks, pushing and holding the brake pedal results in what I can only describe as a clunking noise. Sounds a little like hot water pipes expanding. The longer/harder you push the brake, the more the clunking.
Turns out the driver's side (left hand drive) brake calliper piston is going bad. Repair will cost me €350 including changing the piston and flushing the brake fluid. Dealership says the calliper is more expensive for the diesel engine as the heavier engine up front requires a bigger calliper piston. For the Cooper S model, the price would have been about €250.
Not changing it means at some point the brake is likely to bind.
Hope that might help someone.