I have a 2014 Chevrolet Express cargo van with 66,000 miles on it. Occasionally at start up there is a knock under the hood. One knock, really loudly. The first time it happened I thought someone had hit me. It sounds like it’s coming from the passenger side. The only thing I can compare it too is someone stuck a really large bottle of shaken up soda under the hood, and when I start the van the top explodes and hits the underside of the hood.
Have you checked for a mark under the hood, say, above the oil dipstick?
How about a broken motor mount?
Sounds to me like a possible misfire. You should be aware that the ECU will not store fault codes when the circuitry is in START mode. Ergo, that may not be any help. Although checking for any stored codes is always a prudent first step.
While I realize that your spark plugs probably aren’t “due” at 66,000 miles (check your owner’s manual in case I’m wrong), changing them will give you a good excuse to “read” them. Sometimes that can provide really good clues. I’ll post a “link”, but if you’re uncertain just post photos and we’ll try to help.
It might also be part of the starter motor assembly hanging up, specifically in the Bendix assembly portion. Or it could be damage to the starter motor gear preventing clean engagement when its position is in a particular spot. Or even damage to the ring gear that the starter motor turns to start the engine.
How often does it happen? If it’s often enough, perhaps you could have a friend start the van while you listen to where the knock might be coming from.
One last question. Have you had any work done on the van recently?
Oldtimer had a really good idea too. Aggressively shake the engine manually (with it OFF) and see it anything is going bump.