A mechanic's stethoscope is great for this type of work. You can substitute a long metal rod, for instance a long screwdriver, as a stand-in. Put the tip against the water pump (prime suspect in my opinion) and push your ear against the other end, and the screwdriver will transmit the noise from that component to your ear. Move it around to other accessories and compare the noise volume.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC