Strange Knocking Sound Coming From Tank?

[StevenF]

my tank makes most of the knocking noises on a night after the tank lights have gone out. I wonder if it's a heat thing ... things cooling down? the tubes get really hot
 

 
That sounds like thermal contraction.  When things heat up they expand.  When they cool down they shrink.  Most of the time the change in size is not noticeable.  However occasionally you will hear light pop  or click as it expands and shrinks.  I suspect aquarium heaters could also make the noise when they turn on and off but I have never heard one do it.  For filters and pumps the only sound they could make make would be a vibration induced buzz.

 

[Asteria]

Okays so today when I was observing my tank I also heard a sound that is somewhat similar to what your describing. 
It was like rocks clanging together or a slight knocking noise. 
It was my angels at the surface. They were feeding? Or gulping air? I don't really know for sure but the sound was like a knocking sound really loud as if they were throwing rocks at my glass. 
Next time take at look at the surface and see who's there. 

 

[Akasha72]

interesting Asteria ... next time I hear it I'll take a look

 

[Bazouteast]

MTS - Malaysian Trumpet Snails.
Canister - External filter
SAE - Siamese Algae Eater (fish)

Ah.......TOT!