1990 Buick Electra My car won't stop squealing

1990 Buick Electra mileage: 120,000. My car won't stop squealing, especially when it rains, but recently when it hasn't. I've gone through two belts in the past four years. The last belt was quiet for half a year because the mechanic told me to replace the water pump, which I did (under warranty). Their explanation was that the belt was too dry, so I believed them. The service advisory poured a little water on the belt to demonstrate, and instantly the noise was even louder. After the demonstration, he applied some silicone lubricant and the noise disappeared. I don't believe that this solved the problem because the noise came back again. A neighbor told me that the tensioner might need to be replaced because the spring could be worn out. I brought that up with the service department, however, they said it was fine. Is there anything else that I can check to solve this problem?