I don't know what it is with garages. But I can't remember ever taking any car in to be repaired where I told them what the specific problem was, and they didn't automatically "cop" an attitude. It's like they can't believe any "civilian" can diagnose what the problem is. (I guess they figure if you can diagnose it, you would fix it yourself).
Then when it turns out that you're correct, (just to cover their butts) they will always tell you that there was alot more to it than just the problem that you mentioned.