From AAL MD-80 MM, ATA Chapter 29-00-00-0 (Description and Operation)
(1) A backup means of pressurizing all systems is provided by one, electric, motor-driven, auxiliary pump and
>one hydrau-lically operated, power transfer unit.<
The electric motor-driven pump pressurizes the right system only. The power transfer unit transfers power from a pressurized system to a depressurized or sufficiently lower pressurized system.
(3) The power transfer unit mechanically connects the two systems and enables hydraulic power to be transferred from the higher to the lower system (higher pressure side operates as a motor and lower pressure side as a pump) and supplies a flow of hydraulic fluid at approximately 8 gpm and pressure that may fluctuate within 2000 to 3000 psi (13,800 to 20,700 kPa). The unit is controlled by a single motor operating two shutoff valves, one in each system.
>Operation is controlled by a separate switch located on the hydraulic control panel on the First Officer's Instrument Panel.<
The motor operated shutoff valves are also connected electrically to the low level switch on each reservoir, the shutoff valve will automatically close if either system reservoir is below 1-1/3 quarts (1.26 l). The unit and control valves are located in the left main landing gear well.
AAR90 is quite correct in that the switch (S1-236) on the F/O's Instrument Panel is labeled "TRANS" with "ON" and OFF" positions.
The actual nomenclature (To maintenance) of the unit itself is Power Transfer Unit. I don't have an operating manual handy for that fleet to see what it calls it...
At least that's what the maintenance manual and Lamm manual told me. I don't feel like walking across the base to check "The Okie Blueprints".
I would say that whatever we (excuse me, I, ??) want to call it, the hydraulic system is the source of what you are hearing. I don't really have a problem with either way of reffering to the unit in question. A DC-10 and MD-11 make the same sort of noise.
"Eagles soar. But weasels never get sucked into jet intakes.."