That's an interesting device for sure. I'd always seen ground loop isolators on the audio connectors themselves rather than the power supply, something like this: https://www.amazon.com/Isolator-BOSS...dp/B000LP4RMG/

All they really are is a very small transformer for each channel that has equal windings on each side, so there's some slight signal loss (like 5% or less). Since the ouptut side of the the transformer is not electrically connected to the original ground anymore, the ground loop is gone. I think it might also shift the phase of the audio, but as long as it is being done with both left and right channels, it won't affect anything that is audible. The whole thing only works because audio waveforms are essentially AC voltages.

To tape, yes, aside from the filtering that is the pulsing.

To be clear it’s not an actual helicopter just a similar-ish beating sound.
I am intrigued by the introduction of various filtering devices the existence of which I was unaware of.

Yes I understood you meant a helicopterish sound and I took it to be the low pulsing sound in your audio file. But I wanted to confirm in terms of the actual sound. Hence my filtered file.

The filtering I used was just plain old fashioned EQ.

I’m interested in the usb dongles.But which should I buy? Does each work on different audio errors?
I’m reading reading and investigating.

If you planning to do just a simple dongle, the IO Data one seems to be the best from a performance standpoint (from what I can see online, I don't know it) but also has mostly/only Japanese instructions and a CD-ROM installer. I have tried the cheaper dongles and they definitely stink.

This is not an expert review but he compares it to other dongles. I shared it at the time mark where the review actually begins - https://youtu.be/flc8pOWm6YQ?t=117

I'm not an expert but just for the sake of ideas, I'm wondering if your helicopter sound is because your linear audio head is out of adjustment and picking up part of the video signal or control track off the tape and amplifying it. (maybe that's bee mentioned). You can look up how to adjust that online. It's not complicated but I don't want to speak so far afield from my knowledge. It's worth some investigation. If you're hearing the same thing off a Hi-Fi stereo tape then that's probably not it.

To the OP, you maybe just wasting money, Highly likely none of the mentioned devices will solve your helicopter sound problem as it is clearly not a ground loop problem.