Update -

I took a look at nearly everything on the left, front side of the cabin and could not find anything that appeared loose. So, I'm baffled as to how this noise persists. It's not always there, but when it is, it's annoying. My most recent suspicion: Maybe the wind outside is causing a rattle/fluttering noise that sounds like a rattle as it passes over the driver side rearview mirror?

Thoughts?

Took it to the dealer finally and they figured out what was wrong. The aftermarket battery that was installed by the BMW dealer that sold me the car was not properly being held down, and was moving around a bit, causing the cricket sounds.

What blows the most about it is that they had to add a couple washers to hold it down, and didn't cover that under warranty, charging me a half-hour of labor due to the fact it was the BMW dealership who did it. I went to that dealer, and they refused absolutely to cover the cost (about $57) even though they had performed the work that caused the issue.

At least it's fixed now.

EDIT: fixing my fat fingers.

Had similar sound coming from right side of dash. Finally decided it was from glove box but after I emptied glove box had the same noise. Finally I left glove box open and drove and guess what? No noise!
Apparently the latch itself is making noise. Haven't fixed it yet but should be pretty simple fix.

Derp.
Think I figured it out, and don't know why I didn't think of this earlier. I think the rattling sounds are coming from the windows. Since they're kinda just up and not braced all that much against the car frame at the top, they rattle a little bit at higher speeds. Was hard to determine since the Z doesn't exactly have the greatest sound damping in it as stock.
While I was driving home the other night, I noticed the rattle again. But also realized that it was coming from the passenger side too. Was hard to pick up on that, since the road noise was making it hard to hear from that side of the car.

I have the same problem except it's at all speed ranges.

The weird thing is it goes away when my wife is not in the car...

Solution: dont take the wife...genious I am:facepalm: