Diagnosis
That's usually the sound of refrigerant flowing through the coils, especially right after startup or during a defrost cycle. It's often normal, but if it's a new, very loud sound, it can indicate the unit is tilted.
How to Fix It
First, check that the dehumidifier is sitting perfectly level on a firm floor. Use a small level or just eyeball it front-to-back and side-to-side. If it's leaning even a little, the refrigerant and condensed water might not be draining correctly through the internal lines, causing noisy flow. Adjust the feet if yours has them, or shim it gently with a thin piece of cardboard under the low side.
If That Didn't Work
If the noise is more of a loud knocking or rattling, it could be a loose compressor or fan assembly. With the unit off and unplugged, you can gently press on the side panels to see if it changes the noise when you restart it. If so, the cabinet might need tightening.
Heads Up
A constant, hissing sound is different and not normal—that could indicate a refrigerant leak. In that case, shut it off and call for service.