Peerless Boiler proper pressure causing flushing sound?
Air trapped in the lines. There is usually an automatic air vent in the system but not always. It should be mounted to an "air scoop" or air separator which may also connect the expansion tank. Though that setup can vary. One name brand air vent is the Hy-Vent. If you don't have one, you should have one installed. If you are handy, install it yourself.

And this could be a sign of bigger problems as air doesn't magically appear in a hot water system. Have you noticed any water on/near the boiler? Your expansion tank diaphragm could have failed and air has leaked into your lines. You can test that with with a tire gauge as the fitting is the same as a car tire and is under a cap on the opposite end of the pipe. CAUTION! Let the system cool down a bit before you do this otherwise you might get sprayed with scalding water.

Another thing to check for is leaks around the baseboards and piping. That could be admitting air into the system.