noise from expansion tank

hello all. i have a single zone hot water heating system with an external coil. the circulator is continuously run for domestic hot water. it appears that air may be getting in 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 radiators i heard nothing from radiators or boiler. the next day i heard some bubbling noise from the radiators when the mech zone valve opened. the noise became progressively worse each day. i have since bled the system again, and the radiators are much quieter. however, i now hear a gurgling/bubbling sound from the expansion tank each time the burner comes on. 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? 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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