Re: ok, gang, me needs help! quickly, if possible
Euro - I agree with Craig re the WP. Remove the belt and spin it. it should feel smooth not rough. It may also feel tight, as in not able to spin freely on its own. This is OK. Also, grab the WP shaft and try moving it up & down - there should be very little play in the shaft. The old WP that I replaced in my '98 actually had a metal impeller, so, I think you earlier version probably did, too.
I don't think the hoses will firm from coolant running thru them - they already have coolant in them. They'll firm from heated, pressurized coolant.
I've changed the t-stat housing without draining the block, only the radiator. You can reuse it if in a clean container. Before you loosen the t-stat, place a plastic bag over all the pulleys and belts, and then some cloths, diapers, bedspreads, what-have-you, over the plastic bag and under the tstat housing. This will keep everything dry when any coolant is freed from the housing.
Your plastic tstat housing lasted a longtime. My first one lasted over 90k miles. Don't gamble on an AL housing - IMHO they are too stiff and don't contour to get a good fit to the head, and will leak.
I'm thinking the noise that you heard was the plastic housing cracking and the expulsion of the preasurized coolant.
1998 528i (aka, best car I ever owned!)
D-Con Cache on Fuel Pump Cover