First of all, thanks for all further advice! Dodgetboy, were you spot on or what!
And for Prine, I did put in a roller chain only a year after I got the car, the design of the original drive system with that nylon (!) pulley plain scared me the first time i had the cover off, no way I was gonne keep that in there, no sir!
Ordered a new water pump today, turned out a shop nearby (does mostly chevy and AC delco stuff) could get one for wednesday. A bit dearer than e-bay, but at least I didn't have to wait two weeks to get it...
So, pulled the pump today, expecting to discover something at least mildly shocking and horrible... but the pump was fine!! what the...????
Turned out the crank pulley was loose! :banghead:
Not much, pulling on it it seemed fine but rotating it, it would move and hit against the bolts. This explains why the noise was only there when the engine was idling roughly just after start-up. In stead of turning evenly and exerting an even pull on the pulley (thats why it's called pulley, isn't it?) forcing the bolts towards the edge of the bolt holes, the uneven idle would jerk the pulley causing the pulley bolt holes to knock against the bolts! (looked like a couple of bolts were missing too)
Well, even though it wasn't the water-pump, it led me to discover an issue that could really have hurt if left undiscovered! In addition to that, there was a small leakage in the gasket causing some seepage through one of the bolt-holes, doesn't hurt getting that fixed as well!
Just hope I can cancel the order for the water-pump tomorrow, if not, I'll at least have one in spare!
All in all, I'm not complaining, at the end of the day, the good old Dart is getting better and better everytime I do something with it!