Originally Posted by sailvayu
Soundown makes good material for this. Most lead has been replaced with dense plastic material. The best is when the dense material is sandwiched between two layers of foam. This allows it to move some thereby absorbing the vibrations. Make sure whatever you pick is resistant to oil
fumes. I have seen a lot of the cheap stuff and materials not meant for engine compartments fall apart making a real mess and it can be hard to remove. Also be careful to block any and all holes or opening into the engine compartment no matter how small. Sound waves particularly the high frequency ones will escape even small openings.
In a comparison test of various sound insulation
products the Soundown came out as one of the best rated. The test was several years back so there could be new products out that compare but I ordered the Soundown 2" direct from the company in Ft Lauderdale. Am happy with the product and support from the company.
They make a multi layer product with a heavy, lead substitute barrier sandwiched between layers of foam absorbent. Available in different weights and thicknesses.