1998 C280

So when I pulled up to a stoplight the other day, I noticed my AC system was making a really strange noise. About every 3-4 seconds, there will be a thump noise, which is loud enough to be felt through the dash by my knee, and most of the time, this thump is accompanied by a water droplet sound. The AC seems to be working fine, blowing fast and cold. But this sound is very annoying when I want a quiet ride home.

Anyone ever have this noise before? or have any idea what it is?

Thanks in advance.

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