What could be causing the humming sound in the hot water radiator

It maybe that air got into the system and this is preventing the heating system to fill up properly. You may have to remove the air from the boiler itself, there should be a valve to enable you to do this. Replacing the pump seems to be either coincidental or unecessary. Also, when draing some or all of the radiators down it is advisable to put a chemical in to prevent any corrosive effect from taking place. This may have a minimal bearing on how hot the rads get, clean water in the system can encourage furring to take place and therby reduce the pipe thickness, again preventing enough water through to the rad. Lastly, if the rad or system is old, you may have some thick sludge which will prevent the pipes and/or rad from getting any hot or cold water at all. depends on how thick the sludge is. The sludge could have been disturbed when the system was drained down. To remove the sludge you can buy a chemical remover and put it into the rad which is worst effected and turn the heat on, then most suppliers recommend you drain the system and put a chemical in to prevent this again. Finally, I had to hit one rad because it was located in a room where i could not get to the valves. I kept on tapping it with a hammer to encourage the sludge to move on and into the outflow, with patience this did work but it did take 30 -40 minutes of tapping and bleeding.

Other than looking for hidden bleeders (i.e. I have one where the baseboard cover needs to be removed to reveal the bleeder) and bleeding any bleeders that you can find, you may need to swear at whoever put in the new radiators and retrofit bleeders where they are missing.

There is often an automatic bleeder/vent on or just after (in which case it's often on a big cast iron "air separator" that may have an expansion tank hung off its bottom) the boiler, since in theory any dissolved air will be most prone to come out of solution where it's hottest - if that becomes clogged or otherwise inoperable (they don't last overly long) air may migrate elsewhere more than it would if that was working. Sometimes people see a dribble from these and close down the cap on the "tire fill valve" rather than replacing them - which renders them inoperable. I think it's bad practice to depend on that for bleeding the whole system.

Given how anemic circulator pumps are when faced with air bubbles, the only other way of getting that loop clear would be to open the boiler fill and open a drain on that loop, so that the fill water can push the bubble through to the drain - but then you have new water in the system, and some air will come out of that.