1997 Honda Civic Only does this when wet outside

Here is the problem. A couple of months ago, I got new tires and my 120,000 mile service. Nothing was wrong before this. Soon after that, I noticed that when it was raining, my car started to make a loud sort of chugging noise that seemed to be coming from underneath the car. It would vibrate very hard and sort of chatter - I could feel it very strongly right underneath the driver's seat and would vibrate the console in the car. This would usually start once I got to about 20mph and continue until I came to a stop. It would continue to shake and chatter for a few seconds after I had made a stop. I also noticed that when I was driving it felt as if something was pulling down on the car (ie it felt like it was dragging and would not pick up speed quickly) I took the car in to the tire store and they found that one of the tires was defective so they changed that and also realigned the tires - they were also not aligned correctly. Still the car made this noise - again, only when it is wet outside (either rainy or slushy) Took the car back into Honda, they found one of the belts was loose. Tightened it, but still the car makes the noise. I'm getting a little frustrated that nobody seems to know what the problem is. It is also hard to diagnose because it only makes this noise when it is wet outside. It's a little scary when I'm driving because it feels as if the bottom of the car is just going to fall off.