weird noises from circulator pump.

Perhaps a little more information. Other than that, maybe it is trying to tell you something.

The Circulator pump is located in the best place on that boiler to vent air from the system. The built in air scoop of that boiler is at the hottest location and the pump is causing the lowest pressure in the system to happen in the same place.

When you get the new expansion tank, you want to add a few valves to make servicing and testing the tank easier in the future. See illustration below:

In order to get a proper air pressure reading on your diaphragm type expansion tank, you must have no water in the tank and it must be disconnected from the boiler pressure. If you have 26 PSI boiler pressure connected to that expansion tank. The water in the boiler will enter the tank and compress that air to the same pressure as the boiler pressure.

Your Expansion tank may just be low on air pressure. If that is the case you will have some water in the tank at 12 PSI when in fact you should have no water in the tank at 12 PSI pressure.

Are all your radiators the copper tube type with the aluminum fin?  If yes, and there in a series loop, then you can purge the air from them in the basement.  

1. Place a garden hose on the purge station hose bib opening A and direct the other end to a floor drain, laundry tub or outside.
2. Turn the power off to the boiler B and make sure the zone valves are all closed.  We will be using the zone valves to isolate each zone one at a time.
3. Close the isolation valve on the purge station  C
4. Observe the gauge pressure on the boiler D  Keep it at 25 PSI or below during this purge.
5. Open the boiler fill tab E to allow for fast filling.  Refer to #4 above and close that tab if the pressure rises above 25 PSI.
6. Open one of the zone F, G or H valve manual levers to allow water into that zone only.  Leave the other 2 manual levers closed at this time 
7. Open the purge station valve A and let the water force purge the air from the zone.   
8. If the boiler pressure drops below 15 PSI during a purge, then close valve A and allow the boiler pressure D to increase to 25 PSI. 
9. Once you reach 25 PSI on the boiler gauge, then open valve A again. wait about 1 minute after the last bubble of air leaves the hose.
10. Repeat steps 6 thru 9 on each zone valve with the next valve open while the other two are closed.  
11. Once you have all the air purged then release the fast fill tab E and close valve A.
12. Place all the zone valves F, G, and H manual handles in the automatic position.
13. Open valve C
14. Turn on the boiler switch B and operate one of the zone valves by setting the thermostat higher that the room temperature by 5°F 
15. See if the water circulates the heat across the entire zone all the way back to the boiler return.  
16. Repeat step 15 on the other two zones.  If any of the zones do not circulate the hot water from the supply back to the return then you must purge the air from that zone.  

Notes.

1. The time for a thermostat to open a zone valve may be as much as 2 minutes before the valve is completely open and the burner and pump start to circulate any water.  
2. The time it takes to heat up the water in the boiler to know if it is circulating may take 5 to 8 minutes before you have hot enough water to tell if the first zone is circulating properly.  
3. The time it takes to get the hot boiler water to circulate each zone may be as long as 3 minutes depending on the length of the pi[ping on that loop.

What I’m saying is after you believe you have all the air out, it will take time to verify that it is in fact working properly. So do not get frustrated if the water takes too long to heat up.  The only way to tell if a zone is not air free is if the boiler temperature is above 150° leaving and you are not getting at least 130° returning and that may take 15 minutes on the first zone tested.