transmission or engine fault

hello everyone,i,m looking at selling my 3.9 after the purchase of a rr,but before i let it go i,m keen to solve an ongoing problem which is-when slowing down and braking i,m getting a slight clunk from somewhere under the front/middle of the vehicle.the only clues i can give you is the rpm settles down to 750rpm then just before this clunk happens jumps slightly to 1000 and drops again?also when this clunk happens the brake peddle goes down on its own.at first i thought possible transfer box?but now i,m wondering if its engine related?i,ve checked the obvious props/a-frame etc to no avail.any ideas

wierd one that, if the brake pedal goes down by it's self- wouldnt that have something to do with vaccum possibly??

Stu

sound good mate but what controls the vacuum?and how is it tested.cheers