I have a Thomas dual head rocking piston aerator in my pond. It still works very well with good air production. It has gotten very loud though. I've replaced the seals,flappers and cylinder rings but no change. Any suggestions? It may have gotten wet, but the motor is ok and not the noise source I believe. Any help is appreciated.

Is it a mechanical, metal-on-metal noise? Could your air filter have developed a hole, or leak?

It's more like the metal on metal sound maybe. I've tried it with and without air filter. The sound changes but it's something else.

Okay, is it more or less a continuous, scraping or screeching type of noise, or a knocking sound?

Knocking

Then I would suspect either the eccentric bearings, or more likely an interference/wear issue with the piston and cylinder head, or cylinder wall.

You need to dis-assemble and look for signs of damage, or possible ingestion of foreign materials. I had a Thomas that broke the screw that holds the piston together, and while it still ran and pumped air, it was scary noisy. Did you replace the cylinders when you had it apart? If so, make sure the countersunk screws in the middle of the pistons are tight, and pulled down all the way. Also, be aware that the piston top only goes on one way..

Here is a thread that deals with a Thomas: http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=390615

afterthought...any chance you have a valve reversed?

Thank you for your help. I'll check it out tonight.