I am having a similar problem. I replaced a bad fan clutch that was roaring. I got an O'Reilly's fan clutch from Murray p.n. 2786. I went back to O'Reilly's when it also roared even though brand new out of the box. They only had one part listed for my 2003 4.3L and this was the right one. I was hoping they carried two diff. parts like Groundunit found out about Auto Zone. What is the best way to test the fan clutch to see if it is defective?

 

if you look on the auto zone website they list a fan clutch for ac or without ac. I think what they're doing is "upgrading" the cooling fan. I don't know if this is due to a tsb or what but it seem as though the manufactures think that the heavy duty one, which engages more often than the original one is the way to go for these.

 

I looked at the Auto Zone MCK 1001=w/o Heavy Duty heater and the MCK 1002= with HD heater. These are kits that include the water pump and a fan clutch. The 'with and w/o Heavy Duty' heater phrase is referring to the water pump and has nothing to do with the fan clutch. If you look at AZ's listing for the fan clutch by itself, it shows only one fan clutch.