Constant Knocking Noise from Pipes

Hello All - Newbie here with a diagnosis question.

I live in a 3 story townhouse condo. I am getting a constant loud knocking from my pipes. It occurs every 3-5 minutes for anywhere from 5-8 seconds. Unfortunately, I cannot isolate a certain pipe or area. It doesn't knock in any relation to turning a faucet on or off, flushing a toilet, or using the shower. I have tried to purge the lines to solve water hammer (I don't think this is it because it doesn't knock when I turned off the faucet, but tried it anyway since it was a quick and easy solution)?

Any thoughts on what could be causing the issue? TIA

Did this just start? Do you have hot water heat? Could it be the heating pipes expanding and contracting and the noise is caused by a tight restriction when the pipe goes through something? Could it be coming from a neighbor? If forced air, do you have a humidifier? It could be turning on and off. What about an ice maker? do you know what the pipes are made of? Copper, cpvc, pex? Do you know what your water pressure is? Sometimes a PRV can make some noises. What about a check valve? Got one? Sometimes they are on the water meter or close by. Do you know if you have one?

I will try to address all of those questions.

Heat is forced air, not hot water.
Could be coming from a neighbor. Unit on one side is unoccupied, haven't been able to ask other neighbor.
I do not have a humidifier built into the forced air system.
I do not have an Ice maker.
My pipes are copper pipes (at least the ones I can see in the basement, and the shower downspout.
Not sure if I have a PRV or what the pressure is. Didn't see one there. Unit is 35 years old, I don't think one was installed. I don't have a water meter as the water is common (not individually metered).

The problem started in the spring, then seemed to subside over the summer. It has fired back up over the last month to 6 weeks.

If you have long runs of pipe, the length will change measurably more than a shorter run. It could be the pipes expanding and contracting. The difference of the water in the summer between running hot through them and ambient is likely a lot less that as things cool off, meaning a bigger difference and more length change. You may be noticing the pipes making noise after running some hot as they then cool off to ambient. If any are running through unheated or limited heat areas, they may have a long ways to cool off.

More people will be around this evening...you may get some other ideas.