air bubbles from my maxi jet with a hydor flow ?

i have a a hydor flow on my maxi jet 1200 every once in a while when it spins around i get little air bubble that come shooting out of it

has any one else ever had this problem i have made sure the thing didnt have any air in it ive turn it evey wish way under water and it isnt to close to the top?
 
Yeah mine do it too. My guess is that microbubbles collect within it and then get released all at once.
 
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