I bought my 2006 roadster knowing that it had a pretty loud whistle with the top up that starts around 40 MPH and eases up a bit (still there) around 65. When the top is down....no whistle. I wasn't too concerned figuring that I could zero in on the source. Original owner said it always whistled. First thought was the seal along the top of the windshield or around the side windows. Its one of those sounds that seems to come from the front of the interior but most of the time seems to come from anywhere. Here's what I have tried to narrow down the cause:

• While the whistle is sounding...put both windows down....no change
• Tried alternately opening either door.....no change in whistle
• Took an old necktie and laid along top of windshield seal before clamping down top....no change
• Tried tie between lower rear edge of top and the tonneau cover...no change
• Attempted to lay plastic wrap (Glad Wrap) over the top frame above windshield down onto top of windshield.....still whistled and pretty quickly blew loose.
• Looked for holes/gaps in firewall....nothing obvious
• Got a mechanics stethoscope that came with a rubber horn attachment to listen for air leaks while wife drove......could not zero in a sound increase in any given spot

Am considering:

• the incoming air source vs an exit vent, valve relief...not certain how that works.....maybe whistles when interior pressures build up at speed?
• Possibly a bad air filter or fit under engine cover?
• Maybe bad cabin air filter?

Any ideas? Has anyone experieced this? Found some old posts that never mentioned a resolution. I'm running out of things to try. Any thoughts appreciated........great forum.....have learned a lot reading thru it all.