Transmission shift cable
Transmissions can make a whirring noise in reverse, as the gears are worn in to work best in Fwd similar to an automobile transmission. If there is an irregular grinding noise, I'd have it looked at. Worst case, your transmission is (becomming) toast, check the tranny fluid for telltale signs (level, contaminants, metal shavings). My shifting cable froze up from saltwater in my bilge splashing up onto my shift cable. I removed the cotter pin connecting it to the tranny & it still didn't budge. I sprayed some protectant/lubricant onto the cable where it goes into the housing, then worked the shifter until it moved freely again. If your cable is too corroded, you may need to replace it. Good luck!
