What sound frequencies does a circular saw emit

I wouldn't worry so much about blades flying out, even when using a cheap harbor freight saw. What is more likely to happen is the motor will burn out quicker than a quality tool. I've bought many harbor freight tools for "time to time" use. I've got a drill press that's lasted quite a long while ... granted I only use it a few times a year so I got my money's worth.

I think you'd be fine with the cheap saw as long as you're not going to use it all that often. The blade won't come flying out but the motor will probably burn out and fail long before the DeWalt motor would.

It's probably just the pipes expanding and contracting under temperature changes. They can bind on an anchorage, then suddenly break loose. The floor and pipes do reverberate, amplifying the sound. The tension release and sudden movement can be very small, yet the resulting sound rather startling.

If you can identify where the pipe is hanging up, you can replace the anchor with one that has a plastic liner that is resistant to binding. Or simply loosen the anchor or remove it, wrap the pipe with cloth tape, then reinstall the anchor.

It can be difficult to locate all the pops and creaks, they seem to come from nowhere sometimes. You should at least be able to quiet the worst offenders.