iMac's fans make awful noise
Your fans are running at speeds appropriate for that model. Even using a stethoscope on the rear of an iMac, good techs have been confused between hard drive and blower noise unless you can change the noise by increasing or decreasing the fans or the HDD.

Rhythmic blower ticking is one of two things:


Failing or failed bearings in the blower.
Foreign object (paper, hair, other) lodged in the blower.


Choice #2 is less likely since the airflow around the unit usually precludes things getting stuck there. Also the design makes it unlikely things large enough to make noise or stick can enter the blower area.

I wouldn't recommend shaking an iMac (running or not), but tipping it on a side or upside down with a friend's help and then powering it on would let gravity and the airflow dislodge all but the most unlucky piece of debris from a blower. Be aware that succeeding to move debris may land such debris in a worse place than a blower, so do use caution. 

If the bearings are the cause - you might try a fan control software to run the fans faster (sometimes higher speeds are less noisy) but do get ready for a visit to the shop as bearings don't fix themselves and the noise will get worse as it wears. A technician is the best person to get debris out and truly resolve a ticking noise whether it's a hard drive, optical drive or a blower.