306-diesel knock????

Hi,My car is a peugout 306 and I recently had to get the air conditioning fan belt removed and replaced as it went into shreds.After the mechanic replaced it,the car returned with a terrible noise when started up-it actually sounds worse than a tractor.Shortly afterwards,at times,car wouldnt start but not all the time so I took it back to the mechanic and he replaced the heater plugs but this didnt solve the problem so I then replaced the battery-still no better only to take it back to the mechanic and it turned out it was a loose connection on the starter due to changing the fan belt.The car has just been through mot and had lot of money spent on it and passed,the noise is still there as bad as ever.My mechanic says just a noisy engine but a friend said it was a diesel knock-would it be dear to fix or better still even worth fixing???Would be most grateful for any help given,thanks in advance.