I am flying out of Palwaukee in the Chicago are, using ATIS all the way and it's working pretty darn good too! Right down to the point of them telling me sarcastically there's a Piper on the runway so I can't have clearance

I cleared Palwaukee, started to transition through O'Hare ( I got clearance) and I start getting this beeping that sounds like Morse Code in the cockpit. Anyone know what this is and what I'm not doing right?
BTW, on behalf of all the people who will be cleaning the seats in my Cessna tonight, I'd like thank the controllers at O'Hare for the near miss. I was .2 of a mile above a 737 taking off. If I hadn't 'stepped outside' for a moment I would have missed it. They never even told me it was coming.
