Whistling sound / slightly rough idle / new owner

[sonord]

Hello...
I just purchased a 1990 964 5 speed w/ 48,000 miles. Everything is fantastic and I'm enjoying the car tremendously. However, today I took the car to 6000 RPM a number of times in quick succession then cooled down and left the car. Several hours later I took the car onto the freeway and noticed several things: 1) a whistling sound coming from the left rear at 55mph and beyond that was especially noticeable under acceleration; 2) at rest the car's idle was a little rough and felt like there was more movement from the ear end than I'd previously noticed; 3) at slow speeds the brakes are whining a little when taking tight turns; 4)when the clutch pedal is released it seems lazy with a dead spot at the top before it engages - although shifting and so forth is normal.
The whistling may be a vacuum leak but I don't know these engines yet and a cursory look provided no obviously detached hoses.
Any ideas or comments are greatly appreciated.

[AZ964]

By any chance do you have an aftermarket "cone" air filter in place of your factory airbox? I have a similar whistling sound from the rear of my car and after a lot of unwarranted concern, it turned out to be the result of swapping my factory box for a cone filter.