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Is there any kind of underwater silent filter I might be able to get? The current filters keep the water crystal clear but are really loud...

Loud as in the motor, or loud as in water-fall sounds?
"water-fall" sounds drive me nuts!!!!
I don't think underwater filters are the best choice for RES due to the large amount of excrement and need to clear out floating left over food.

Yeah, the water falling. That's what irritates me the most...and I'm really good about only feeding her as much as she eats.

LOL
Penelope's tank is in my office so I have to listen to it all day.
When the water line drops below the outflow, I go nuts!!!!!!
...and when I am on a call, people mention it.
So I hear ya! (pardon the pun)
I wouldn't go with a gravel filter...what do you have now?

Her tank has two hanging filters, one is a lot bigger and the siphon thing goes all the way to the bottom of the tank...but I can only raise the water level so much and it's still loud. I wish there was some other way...there is no gravel in the tank. Just some fist zised and bigger rocks at the bottom, and some fake shells.

Hanging filters...there ya go!
They are typically made for fish and make an assumption that the water is at the brim.
With turtles, as you know, we have to leave room for basking areas and such.
The sound of money hurts....but you may want to buy a canister filter - that way the intake of junk and outflow of clean water occurs under water and you don't hear it.
How big is your tank?
That will help you decide the cost to replace the "waterfalls"
I still don't recommend an under gravel filter - first, they are made for tiny fish poop an second, gravel in a turtle tank is NO good.

Look into canister filters, the return line runs a nozzle into water. No water sound. Its a bit pricey but best filters are those

I'll check it out thanks. Canister. Gotcha.

Way back when I used the hanging filters, marisa and I can up with similar solutions. She used a piece of plexiglas sort of like a slide for the water to travel down and I used a piece to create a flap to direct the water down. The original here: viewtopic.php?t=1405

That said, just get a canister like DB4L says