Mysterious fluttering sound

My 45k mile 1998 996 C2 has just started developing a fluttering sound from the left-hand side when cruising along between 55 and 65mph. It's pretty loud and can be heard clearly. It only does it maybe once every half hour and is silent the rest of the time. I've checked the undertrays, wheelarch liners, wipers, wing mirror, glass rubbers and strips, everything, and when the car's sat stationary everything's solid and firmly fixed and it looks and feels like a new car. Does anyone have any ideas?

You say 'glass rubbers and strips'. Did that include the front and rear screen seals? It was a common problem on the early 996s. Other than that, I'm guessing. Sun roof? Exhaust heat shields, although they rattle and buzz, rather than flutter? Do you think it is front or rear?

Thanks Richard. I've been around prodding the seals on front and rear glass and they look like new. No sunroof on my car. The exhaust heat shields seem fine, but as you say, that would be a metallic tappety noise. This sounds like when a roofbar strap comes loose and flaps. It's coming from the lefthand side only, but I'm not sure where exactly - somewhere between the wing mirror and rear window. I'd get someone to drive alongside me if it was more regular, but on an hour's drive it'll do it maybe twice for about 30 seconds at a time.

Long shot... sound of very early movement of exhaust baffles, maybe? Or leaf/foreign body caught some where underbody/wheelarch.

This isn't a flex, but I was sharing the track with a 919 and a 917 amongst others, so lots of cars running big fat slicks. I arrived home with marbles stuck to my tyres. It's possible one has got stuck somewhere. It doesn't sound like a rattly exhaust, but it may be. I'll make sure I post back if I get to the bottom of it.