Loud engine fan on cold start

The last time my car was in the shop (1995 Crown Vic, 4.6, 100k), my mechanic noticed one of the fans on the engine was not working properly (he said "I could stop it with one finger") so he replaced it. Now, the secondary (?) engine fan always comes on when I start the car. It shuts off after 20-30 sec and then runs fine and the engine never comes close to overheating. I took it back to him but he couldn't find anything wrong (it never did this before he "fixed" it!) Is there a bad sensor to fix or a plug I can pull to stop people from thinking I have a jet airplane parked in my drive? Thanks!