2002/E46 Noise under car?

One of my friend has this 2002/E46 316i/1.9-8v, only done 47'000miles with last BMW service 2 years ago, but since last november, it start to make some cranking noise when starting in cold mornings, its not constant, just a few seconds of loud clonking, but overall the car seem to be much more noisy than my 2007 model. The noise seem to come from under the middle of the car, may be more towards the front , some one said it may be the Cat Converter.

The car overall is like new and not been driven hard , the other problem is idle rev, when at traffic stops, the engine seems to drop then rev up , this can go on for a few minutes until the computer regain control , the rev counter just goes up to 1'000, drops to 500 then up then down.

One of the BMW place replaced one of the sensor, seem to cured the problem for about 2 weeks then back it came, according to the computer, thats the only fault showing at the time 3 months ago, anyone have a clue what shes looking at to solve this one ?