<<< During a wash or rinse cycle it'll start whistling within a 20-30 seconds of starting up. >>>
Try removing the CHOPPER---re-assemble the filter cover---and run an empty wash cycle (or 2 if needed) to determine if the noise returns.
If no noise---replace the chopper with a new one...http://www.appliancepartspros.com/lg-blade-5832dd4001a-ap4447051.html
If still noisy...
Check all of the WIRES inside the door (remove door panel)
Inspect the wiring from the bottom of the door---up to the detergent dispenser housing
If any broken wires are found---repair as needed.
Wrap the entire wire harness (from bottom of door to dispenser) with electrical tape.
Place a small section of cardboard between the wire harness and the tar-like insulation on the door panel (keeps wires away from insulation).
<<< Pump and motor assembly were replaced about a year ago >>>
What is the part number of the SUMP ASSY that was replaced?
Should be AJH31248604
If the part number was *different*---likely need to replace the WASH MOTOR...http://www.appliancepartspros.com/lg-motor-assembly-wm-4681ed1004b-ap5325099.html
If the CHOPPER and the SUMP ASSY are good & correct---the only possibility that remains---is a faulty Main Board (sticking motor relay).
Main Board failure for this symptom is fairly rare.