My Filter's Making Weird Sounds :/

[Myuu]

My filter is making a noise that seems like there's air trapped in the intake tube. It's been making this noise for a while, but the other night after I did a water change it started making this horrid high pitched mechanical noise. I turned it off and took the filter apart to check the impeller and the rest of the equipment. Nothing was visibly wrong from what I could see, the impeller was spinning fine, etc. I fiddled with the impeller just a bit and now it's making the first air bubbly noise again. It's mildly annoying and I'd like to fix it Any ideas?

 

[Rooster]

Air can be a devil to find... especially on external filters, If you have an air "lock" in a coiled pipe etc it may NEVER work itself free... and the impellor wont live long running in air !
I find that with internals, taking them apart and reassembling UNDER WATER cures them... but if your media has been exposed to air - or you have added fresh media, it may take a couple of days to "burp the system"

What filter are you using ?

 

[Myuu]

It's a Penguin 350. I haven't taken out the media since it started running, which was probably around 3 months ago. In fact, I think it's been making that airy noise since I started it. I always thought that it made that sound because it was new and it would go away. I'll try reassembling it underwater when I get home today.