It's probably a harmonic noise from something loose in the exhaust; loose clamp or bolt somewhere. See if you can make it happen not under a load, parked. If you can, it's probably exhaust related, if not it could be a U-joint.
2016 Chev Silverado LTZ crew cab. Bone stock (until 36K)
Sold 2007 (LLY) Chev Silverado 2500HD Classic