Well, if PC is AC powered and DAC is AC powered and neither have isolated power supply or isolated USB port then ground-loop induced noise is quite likely. As
@Roland68 mentioned only listening test (you hear some ticks, clicks, buzzing, white noise when PC is connected to DAC but noise disappears when PC is disconnected) or measurement - record output of the DAC and check noise floor with and without USB isolator (use Laptop on battery for recording, it will be isolated recorder and will not cause another loop).