Loud banging/knocking while cold starting

[TheBully]

Hi I had an oil change done the other day with Triple QX Sythetic 5W/30 and a Bosch oil filter.  Last night starting up the car in the frost to go home from work I heard a really bad knocking almost like banging from under the bonnet which slowly subsided after about a minute or two.  It did it this morning as well, and its been below freezing when starting up each time, its not like valves ticking it sounds like metal on metal hitting like someone has a hammer tapping the bonnet.  The car runs fine after that its just pretty worrying.  I have a 05 2.0l WRX blob eye about 120k on the clock.  The garage told me they put 4.3l in and it showed nothing on the stick, so they filled it up using 4.7l of oil in total.

[TheBully]

I check it at lunchtime, I think the belts are too loose and that is what could be causing the racket.  They have been squealing for a while while cold but I have noticed the two belts alternator and power steering have been rubbing against each other and the edge of one of them is worn down with dust everywhere.  Oil still seems good like new and my antifreeze is to the top of the radiator and red not just water with no gunk.