The June 2002 issue of QST describes a modification to the 756PRO. Additional power supply bypassing is added to eliminate an adjacent channel noise problem.
The problem has been fixed in the 756PRO-II.
Here are more details:
Cures key-click-like noise (on CW) and rumbling noise (on SSB) due to IF passband coupling to the 8V supply rail. Occurs when strong signals are within 7.5kHz of the indicated frequency.
Remove the bottom cover of the 756PRO.
With the backside of the 756-PRO facing you, locate the DSP-box on the left-side (the shiny-box).
Lift the DSP-box (with care !) out of the mainboard.
C316 is a 100uF capacitor located underneath the DSP module.
Carefully solder 2 wires in parallel with C316, the PCB contact pads extend a bit on either side, and connect the wires to a 1000uF 16V capacitor.
Observe the polarities on both capacitors - the negative side of C316 is towards the front panel.
There's space to mount the new cap using double-sided tape next to the memory back-up battery.
This will improve the bypassing on the 8V rail and should cure the problem.