It's quite possible the source is SMPS (switch mode power supply) noise, either in the Zoom or in the USB port. This can require some electronics expertise to sort out.
Simple stuff to try:
If the Zoom has a wall wart, try swapping it with another wall wart that has the same voltage, connector polarity and GTE (greater than or equal) current rating.
If it's USB powered you could try using a powered USB hub.
Another simple diagnostic test would be to carefully measure the noise level at high sample rate, then unplug any nonessential USB devices to see if the level or frequency of the noise changes. If it does, at least one noise source is the USB power supply of the PC.
Beyond that, you'd need a good oscilloscope to trace the noise to the source.
If you're handy with small scale soldering, you could tack-solder a 1000 pFd capacitor across the power rails of the Zoom supply or the USB 5V/Gnd. Don't try this unless you know what you're doing.
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