Howdy!

I live on the 2nd floor of a 3-story complex with forced hot water, and from 11pm-3am almost every night I can hear repetitive thumping/tapping that sounds like someone is tapping on the wall. The thumping is about 2-3Hz (4-6 thumps per second), it happens in bursts of 3-7 thumps at a time, and continues for 10-15 minutes.

I thought it was my neighbor upstairs, or an animal in the wall, but one of my neighbors on the same floor has heard the same noise. Could this be related to our water system?

Much obliged.

probably . It's called water hammering .Your landlord needs to bring in a plumber to identify the pipe or fixture that needs adjusting .

Be embarrassing if it was just lovemaking and a headboard against the wall. I would sort of talk to other neighbors first.

Air needs to be bled out of radiators or in the basement somewhere, depending on the arrangement of delivery.
When I had big old fat radiators years ago, I had to do it at least once a winter.
Besides, you get more heat without air in the pipes.

Gnargnar,
It is possible that you are hearing noise caused by expansion of the heating loop piping, or, the supply / return piping to your heating units. Most hot water heating systems operate in the temperatures range of 160 - 180 degrees. When this hot water travels to your heat emitters on a call for heat from the thermostat, it replaces the room temperature ( ambient ) water, and causes the piping to expand rather quickly. The piping, typically 3/4" copper, will transmit vibration to everything it touches, often causing ticking, clicking, or even thumping / banging. Every100 feet of copper tubing can expand up to 2" as it gets energy from the 160 /180 degree water, and, the tighter the pipe is held in place, or is routed very tightly through mis - aligned drilled holes, the more noise it will transmit.