Have you a stethoscope? Or a long (like really long), screwdriver? Or worse case, a piece of heater hose about 3' long?
You should be able to 'sound it out', find out/narrow it down fairly close with that.
I can't rule out anything without actually hearing/feeling it myself.
Can't say I"ve seen many, (if any), water pumps go tits up on exploders yet. (but we're a small dealership).

Good luck. Have fun. [img]/phpBB/images/smiles/icon_wink.gif[/img]
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