Eheim Pro 2, Impeller/propeller Rattling.

[Nickpan]

Just finished a partial water change, and turned all power back on. Filter is making an awful rattling noise, in fits and starts (not just a constant rattle like an airpump against a wall). I've had a look 'under the bonnet', wiped green muck off any surfaces, and powered it up. The impeller whirls away, then briefly pauses (slows for a split second) before rattling on.
Could it be that I need to do things before turning on an external filter? Is it because I didn't do the siphon procedure?
It's all second hand, so I'm new to it.

 

[rabbut]

If you didn't start a syphon and allow the unit to fill that way, yes, not starting the syphon is your problem

 

[Nickpan]

If you didn't start a syphon and allow the unit to fill that way, yes, not starting the syphon is your problem

Yep! Whilst you were typing, I read a similar thread that I'd originally recoiled from (absence of punctuation made one of my eyes bleed). I wasn't looking forward to the sucking, but barely swallowed a mouthful. Looks like I've been lucky with the previous 2 water changes, as filter worked fine, but from now on I shall suck on the outflow pipe afore powering up.
God bless internet.