You may have hairline cracked the suction tube and your siphoning air...
The crack may be hard to see.
....Kodiak.


"Cold Ethyl" ethyl@neinspambittenononohowlongcanimakethisbluey onder.co.uk
wrote in message news
Cleaned my tank out this afternoon, and ever since my Fluvel biolife
filter
has been making strange gurgling noises, a bit like water going down a
drain. It's like the power to it cuts out, then comes back again. I've
tried
cleaning the motor and propeller, but it doesn't make any difference. At
the
moment I've dug my ancient fluvel 4 out and have got that running as well,
though there won't be any bacteria colonising it atm. Anyone had anything
similar to this?