Nethawk

I recorded this audio clip with a lav mic hard mounted to one of the passenger side engine mount bolts. When cruising around 2.5k rpm and 40kpa load I can hear what sounds like detonation in the recording. I tried massively retarding the timing in these cells, but the sound is the same. Any ideas what is could be? Piston slap? Sounds healthy at full throttle at all rpm. The sound is most notable about 37 seconds in the audio clip.

Build details:
<500miles on full rebuild
2005 VVT engine
9.5:1 Supertech pistons