Gunshots in the dashboard

I have a '99 Volvo S70 with 68,000 miles. Recently I hit a bump and a VERY loud sound closely resembling a gunshot (.357) came from the vicinity of the driverside dash or inside the engine compartment. I am not easily spooked but this made me jump. I didn’t hear it again for several weeks but it started happening again. Gradually the frequency increased and the volumde decreased. The mechanics could not reproduce it but one day last week when it was happening several times a minute I pulled my mechanic into my car determined to show him that I wasn’t crazy. Unfortunately they had resurfaced all of the roads around his shop and I could not get it to happen. Since then it has happened only once but I fear the worst. Any ideas?

I would first suspect something wrong with the front suspension or the engine mounts.

Motor mounts have been replaced and suspension checked. But thanks for the response