I think the culprit is the valve that you shut off to replace the ballcock. These valves don't get used and the rubber washer gets old. When you close it for the first time in years the rubber washer gets stuck to the seat and ends up partially plugging the water way making the noise you describe. Repair or replace the valve. Stop valves below the toilet are notorious for this.
You also could have put in a cheap ballcock and the flow rate is different. A fluidmaster ballcock or a factory replacement is the best. If the noise happens during flushes then you could try pinching the stop valve down a little at a time to see if that helps. The stop valve is below the toilet coming out the wall.
Also your heating radiator water should not be going to the toilet and potable water system and vice versa. These are two separate systems (even if they use the same boiler) so let us know where the sounds are coming from. For help with radiators, go to www.heatinghelp.com