Just thought I would close the loop and thank you all for your responses.
It turns out that the issue was as @alioth suggested – slightly warped exhaust mating faces. They were smoothed out and appear to have solved the problem.
This is good as I was finally able to take my 9 year old daughter up on her first flight on Father’s Day Sunday. I filmed the take off and the look on her face when we got airborne really made me – and obviously her – happy that I was able to share my passion for flight with her. I let her fly and she did a surprisingly good job, despite not being able to reach the rudder bar. She liked the steep turns 
and asked when we can go flying again…
Good to know!
How nice that your daughter enjoyed the flight so much.
Excellent news :-) May your exhaust system remain true for many years!
