Three other possibilities when it comes to pump noise for the L20-
1. Could there be any bubbles in your system? Bubbles add to pump noise.
2. Could there be a choke point in your system? If the pump is having a very difficult time pushing the water through the loop it can cause cavitation. Cavitation will cause noise.
3. Is there a chance your pump was run while dry and it has somehow damaged the impeller? Some pumps react very badly to being run dry. The impeller blades can be damaged, the impeller itself can become unseated, etc.
How long have you had the L20? Could it have worn out? I have an L35 which is the big brother to the L20 and in all honesty I cannot hear the pump over my case fans which leads me to believe the L20 (which is supposed to be pretty quiet) is not operating as it should be. Its possibly damaged or defective. Just for info I have no high performance/high noise output fans. My 'loudest' one is a 35db 120mm fan so the noise level is not very high.
If its vibrating and making that much noise I would call Hydor (the manufacturers for the pump) at their US number. I had some questions on availability for spare impellers and was able to get ahold of them fairly quickly during standard work hours.
Here is their contact information:
Hydor USA Inc.
9280 Madison Ave, CA 95662 Orangevale, California - USA
Ph. +1-916-988 5222 - Fax +1-916-988-5522
If I remember correctly the person you will speak to will most probably be named Brian. Simply ask for tech support however, they are fairly quick and friendly.
Good luck with it and if you wouldnt mind report back.
Short and sweet the L20 is supposed to be pretty danged quiet unless something is wrong. I would bet there is something that can be changed/fixed fairly easily and inexpensively.