The Pop Off Valve on 98 STS whisles some mornings when the car is first started.

I have searched the forums and found the PN is 3546341 for the Pop valve/backfire valve. I have not seen any information about the installation of the valve or the removal of the old one for cleanling. It appears you screw it out.

Can the valve be removed from the intake manifold without removing the intake. It also appears on my car that the fuel rail must be removed if you can.

Does anyone have any information?

I don't know the answer to your question but, before you go through the replacement process, open it up and make sure the mating (sealing) surfaces are clean. Also see if you can get at the spring and maybe clean it with carb cleaner. Maybe it is just not sealing perfectly, causing the whistle (vacuum leak).

You rotate it 90 degrees counter clockwise, and pull it straight out. It might take a bit of working to pull it out. You'll have to lift the rail out of the way, you shouldn't have to remove the manifold..