Weird Sound

[mrhook]

Ok guys, I have an 04 Sierra 5.3 ext cab z71. I have this weird noise coming from under the passenger side of the truck. Only does it when it rains or in the winter. It almost sounds like a cow moo-ing. I thought maybe the heat shield but it isnt loose. Any ideas? Maybe the cat I was told. How do you test if thats what it is. I thought it was weird that it only does it when it gets cooled off I guess from Cold Canadian Winter temps and Rain water splashing on it I guess. Anyway thanks for your help.

[bill190]

Get a long vacuum cleaner hose, tie wrap it in different locations under the hood, then run through the passenger window. Let a passenger listen as you drive. Then move it to different locations. Try to find where the noise is loudest.