DIYbanter - 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
help.email me at just remove the 5 for spam
purposes. or post message with any help. thanks again.