My engine sounds like it has a lot of marbles rattling around in the cylinders when I roll on the throttle. This happens when I'm climbing a hill and sometimes when I accelerate going around a corner.
This complaint just might be the most common of them all with big Vulcan engines and perhaps other large V-Twins as well especially when air temperatures go up.
What you're hearing is pre-ignition or detonation. Both are abnormal combustion and to keep this description a little shorter we'll just combine the two and call both pre-ignition.
Pre-ignition, as the term suggests, is the ignition of the fuel-air mixture before the regular ignition spark from the spark plug. If the regular spark occurs shortly after the pre-ignition, the colliding of the two flame fronts will cause a pinging noise. Pre-ignition causes loss of engine power and can cause severe damage to pistons, rings and valves.
With the Vulcan 1500/1600 engine the problem is especially severe in summer when intake air is typically 40 or 50 degrees higher than winter air. If you have a stock Vulcan intake you can add another 20 degrees with intake air being drawn from between the (hot) cylinders.
Many, maybe most pre-ignition is not the bikes fault, it's mostly the riders. Lugging the engine (riding at too slow a speed for the riders choice of transmission gear) causes the engine to be very sluggish so the rider rolls on more throttle, dumping a lot of fuel into the cylinders that can't be burned so a lot of that turns to carbon. Eventually the carbon sticking to pistons and valves develops little feathery edges which become hot spots. When the throttle is opened, combustion chamber pressures go up, heat goes up (especially in summer) and those hot spots try to ignite the fuel/air mixture before the spark plug does. The "knock" or "ping" or 'collection of marbles' rattle that the rider hears is a series of colliding flame fronts that are radically changing the ignition timing at best actually trying to force the piston backward if the mixture is ignited soon enough at worst.
Pre-ignition not only causes power loss that the rider can feel but it's also hammering the piston rings. Eventually it can hammer them so hard they break and then you have some very serious problems. In extreme situations a hole can even be punched in the top of a piston.
Now that the cause has been explained here's the fix. DO NOT LUG THE ENGINE.
Different engines like different rpm ranges but for the Vulcan 1500/1600 torque is being made beginning around 2500 rpm, peak horsepower around 5000 rpm. The riders job is to keep the rpm inside that range at all times and higher is generally better. Vulcan engines just love to be revved and redline doesn't appear until 6000 rpm. The limiter will keep the engine from exceeding a safe limit so wind it up, you can't hurt the engine in fact you'll be putting far less strain on bearings and other parts by keeping the rpm high.
But there's no tach on the bike out of the box so how do you know where 2500 rpm is? I have just the thing for you on this Fixit page. Just click the box for your bike model, make changes for the speedometer error ( +10) if you haven't added a correction device and then enter 2500 in the green RPM box. You may be amazed how slowly you've been turning your engine riding in the wrong gear, maybe since the bike came out of the crate.
There is another fix. This is really considered a 'temporary' solution just to see if richening the mixture solves your knocking problem. You'll find it on this Fixit page. If the resistor trick works then I'd suggest saving your dimes until you can afford an after market FI module like a TFI or Power Commander. Those devices will allow you to adjust the mixture to suit your full range of riding conditions rather than just adding fuel at all engine speeds/temperatures.
If your engine has been knocking for quite awhile it might be beneficial to run a dose or two of SeaFoam through it to bust up the existing carbon deposits and start fresh.