air pump making howling noise
 

I have a 02 cadillac deville and the air pump on the front left is making a howling noise when the car the started from a cold start. The sound last only about 45 seconds or so. Once the car idle down, then it stops. If the car is warm, then it will not do it. I changed both check valves. They were clogged up and one was not functioning at all. I thought once they were changed that the howling sound would stop since the air pump would not have to work that hard, but I was wrong. I put another air pump on the car myself and this one is howling too. Any help would be much appreciated. This noise is driving me crasy.