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When driving the car no heavy knock is evident , however, when the brake pedal is depressed firmly for a few seconds ( for example , braking quickly from 30 mph to a standstill ) a single heavy knock can be heard and felt coming from below the gearbox tunnel . Having not had the car for long and not been under it yet to have a good look to try to find the problem I am not familiar with common faults yet .

It sounds to me like when the brake is depressed , the weight of the engine and gearbox is being pivoted forward and therefore lifting the gearbox off it`s mounting and when the brake is suddenly released it returns to it`s original position with a heavy thump .

That`s my theory at the moment until i have a look . Has anyone come across this problem or knows what it could be ? Thanks in advance .

David it does need to go up on a ramp with a person who knows what they are doing to be able to listen and see at the point of the noise when the car is rocked forward to look for play in the gearbox mounts,suspension mounts,and steering mounts........look in the Reccomended suppliers list for your nearest specialist........it could easily be just a bush thats worn through.......or even caliper noise ....sound travels all over the car in situations like that.........Joe