For the past year I've been experiencing a whistiling sound at highway speeds, I originally thought it was a window of sun-roof seal, but then I discovered I could make it go away by a) turning the fan speed up, or b) turning recirc on. I found and old thread on this issue, but no clear fix.

https://www.northamericanmotoring.co...pitched+whin e

I've got an '05 Cooper S, and it didn't have the problem the first couple of years. Whistling appears to come from the passenger side. One other anectodal piece of evidence: At 85,000 miles I decided to start doing some maintenance myself, in particular "Inspection I": Oil, filter and microfilter. When replacing the microfilter I noticed that firewall side of the cover was broken and didn't latch, i.e., I'm not getting a good seal. I wonder if that may be the source (Onset of the issue coincides with last time dealer serviced my car...I wonder....).

 

It does sound like it is the microfilter cover not sealed.

I had the same problem when I changed mine and did not fit the cover on the entire way the first time.

It can be a little tight down there to see if it is clipped in correctly. I took an inspection mirror and a flashlight to check mine after installation.