With winter coming up, I started to heat the house and for the first time the radiator pipes are starting to whistle when I heat and then shut down the system. I have a big house and I need to shut down the heating system regularly if I do not want to blow out my budget. I went in the basement to inspect the system and I saw that the bottom of my boiler is leaking: could this be the reason why the pipes are whistling or is the issue coming from an another source?
You might have air in the pipework to the radiators. You can bleed air from the system's heating circuit (usually a bleed valve is present at the highest point in the pipework)
and from individual radiators.
Details vary depending on which side of the planet you are standing.
You say that your boiler is leaking, but more likely you have water on the floor from your PRV(pressure release valve). Likely you have too much pressure in your system. If you have a pressure guage on the boiler, that should let you know. Somewhere on the pressure release valve should say the maximum pressure before it releases water from the system.
If you are below the release pressure, and you believe the guage is working. Your PRV may be defective. Replace it. It isn't a difficult thing to do, but you have to properly flush the system of water first, then properly refill when you are done.
If you are in fact at or above the release pressure, that would explain the puddle. In that case the question is why. Part of the answer is probably air in the system, as stated in the other question. Bleed all the radiators. If radiators higher up need bled again and again consider automatic bleeder valves/ screws on those radiators.
In my experience, the culprit here is the expansion tank. If it is an old style tank (Big likely strapped between joists in the basement), then you should turn off the system, then turn off the water flow to the expansion tank and drain it.