It could be a bit of noise from the exhaust? I had a similar noise under load when I first rebuilt the engine. Turned out it was the manifold-exhaust pipe joint on the driver side joint 'bedding in'.
It was such an awkward pig to get at I finally got a couple of turns at the nuts after 3-4 months and it seemed to cure the problem
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Daily drive:
2016 Infiniti QX70.....a car that's even more complicated than my last Infiniti FX to modify 
Current 'Project':
VW Crafter CR50 4 berth camper van
Previous 'Projects':
2006 Subaru Legacy Spec 'B' 3.0R,
1994 Ex-MOD Land Rover Defender, now "civilianised".
2000 Audi S8 FL
www.whall.co.uk
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