Discovery II TD5 - Exhaust manifold Problems

I have just recently returned from Victorian High country trip (perth / melb rtn) and on the way back I noticed that when the car was proceeding up the gears a mid high pitched squeal coming from the engine compartment. I looked and couldnt find anything amiss. I put it down to the new Hiclones fitted and passed it off through noise from outside sources covering it up and that it faded once settled in top gear. I have just put the car in for a full service and the mechanics have found the problem, the exhaust manifold has buckled and popped 2 bolts, which they have had to easyout. It got to minus 5 at Mt Freezeout and I did have a little problem starting it one morning which I put down to the wax in the fuel... has anyone else had an exhaust manifold problem on a series 2 TD5, and can anyone tell me what may have caused it.....