Spyder "popping" on lift-off

[droptop]

Brought the spyder for a "spirited" drive yesterday with the top down as it was actually DRY in my normally moist little country.
Anyway, when lifting off the pedal at something around the 5-6k rev mark and coasting in gear back into my lane, I heard a series of pops from the tailpipe telling me there's a little back breathing going on.
Is this normal at higher revs or is my timing out?
No issues starting or running, fuel consumption is good and acceleration is very satisfactory.
I have a series one 2.0 with a single Weber 34 DATR and K&N pancake filter.
Spark is courtesy of points and condenser.
My fuelling is a little lean due to not wanting to start re-jetting the carb as I'm in the process of sorting a set of twins for it.
Am I at risk of damaging anything short term?
I've covered approx. 1500 miles since fitting the filter.

[HFStuart]

The popping comes as standard and I wouldn't have it any other way!