Weird noise after replacing water pump?
 

Just replaced water pump and hoses on my 01 Blazer 4.3L (4l60E tranny), and now it makes a weird noise. It only makes the noise when accelerating betw. 1500 and 2000 rpm's. Kind of sounds likes marbles rolling around. Seems to be louder when engine is cold. Unable to duplicate noise when parked with engine running?

Any ideas?

Thanks in advance.

Possibly the fan hitting the fan shroud?
The lower shroud has 3 tabs on the bottom that lock it into position.
The upper shroud has two locator pins that fit into the lower shroud.
The upper shroud has two rubber spacers that go between the radiator and the upper shroud.