My first thoughts were the blend door acting out too, or a cooling system/heater circulation problem. Does your engine temp operate in the upper range of normal, or has its usual operating temp range recently changed????
So, to isolate the blend door, disconnect & plug the heater control under hood vacuum feed connection & see if the noise stops, if not then maybe begin a cooling system/heater circulation problem.
I've heard of Rangers of this vintage with the 3.0L Vulcan engine with "thumping" noise, caused by a scaled up engine block, causing hot spots that would cause the coolant to boil & cause a thump.
Also water pumps with eroded vanes from using the wrong coolant recipe that aerated & caused cavitation & that eroded the water pump vanes.
Or not changing the coolant on time, such that it became acidic, etched away at the water pump vanes, causing poor coolant circulation & that along with block scale, caused hot spots that would boil the coolant & cause thumping.
Some 3.0L Vulcan Rangers of this vintage didn't have the engine heater coolant bypass hose, that let coolant circulate freely from the water pump through the block, back to the pump while the engine was warming up before the thermostat opened, so adding the bypass hose increased coolant circulation in the block, reducing temps around the cyl water jacket, preventing the coolant boiling & causing thumping.
More thoughts for consideration keep us posted on your findings.
94 Taurus GL 3.8L PEP204A
99 Ranger 4dr 4.0L 5spd Auto
Payload & Tow Pkg
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