odd knocking noise when fan is on

[Jc12]

Hey guys, for the past few weeks i have been getting this slight knocking noise from the engine bay (very hard to explain) that can be heard on and off when the heater fan is on at the lowest setting. I haven't had much luck reproducing the noise but i believe it is almost always linked to when the fan control is on but i wouldn't say mutually exclusive. it seems when i adjust the fan the engine idle moves a fraction which may enable the knock.. anyone have any leads? Possibly unrelated but the other day i started the truck (it was warm) and it had a very rough idle and misfire present for about 20 seconds (no engine light or smoke) then straightened itself out and ran perfect. Never happened before or since..

thanks!

 

[BlackWorksInc]

Is the noise over by the firewall? If I remember right the 3rd gens had a heater valve mounted on the firewall no? Are we talking like engine knock in terms of pre-detonation or a mechanical knocking/clicking/clacking?

Edit-
Bah forgot they're cable operated and not electronic like the newer units. Don't get many 3rd gens at the dealership anymore. lol What's the coolant level like?