A simple trick used by many automotive manufacturer to quiet down an engine is to reduce the induction sound by adding baffles in the airbox. The opposite is true of modders looking for more raspy induction sound.
Since an engine is essentially an over-glorified air compressor, this guy showed us how he tackled his noisy air compressor (and improve on the stock air compressor's rudimentary filtration system at the same time).
You would need a donor airbox preferably from a common car for quick availability of replacement filter, a Dremel, a drill with a metal bit, some brackets and various fitments. PVC fitments would work just as well. If it we up to us, we would also try to shorten the length of the pipes to the cylinders to prevent any possibility of air starvation.