Doesn't sound good, you're at high load there because of the low rpm so, you're gonna magnify internal engine problems that generate noise. Does it do it clutch in or not in gear? That isolates out the transmission. Is it road speed or rpm dependent? That'll tell you driveline or motor. Pull the accessory belt and drive the car for a short bit, that'll isolate accessory drives. I think, if it's internal, it sounds most like a main bearing; it's too deep sounding for rod knock.