noise from boiler

this is actually the way this situation occured.
i have a single zone hot water heating system with an external coil. the circulator is continuously run for domestic hot water. the problem started with gurgling sounds from the baseboard radiators. it sounded as if air may have gotten into the system. i had a "heating prof" in and he filled sysem and bled it from a valve located near the boiler. i live on the first floor and the heating pipes run from boiler, overhead, and then down to baseboard radiators. when he first filled and bled the system i heard almost nothing from the radiators or the boiler. the next day i heard some bubbling noise from the radiators when the burner came on, and the noise became progressively worse each day. i bled the system again. initially the radiators were much quieter, but again began to gurgle louder. upon closer inspection i determined the gurgling/bubbling sound is coming from the expansion tank each time the burner comes on, and the noise i hear from the radiators is actually coming from through the pipes from the expansion tank/ boiler room. the burner comes on at 170/180 and goes off at 200 degrees. could the expansion tank be faulty and somehow letting air into sysem? could the system actually be running hotter than gauge reads and aquastat is set for? or could the hot water be surging as soon as the burner comes on? i believe the expansion tank is one of the newer ones with a bladder. thanks for any me at just remove the 5 for spam purposes. or post message with any help. thanks again.
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