Hi, I think you might have to do a bit of experimenting with the mechanic here/hear...! I spent years working in very noisy machinery spaces on submarines and for a long time I was convinced that hearing protection would reduce my ability to hear the turbines etc as any defects evolved. Then I discovered that actually the right hearing protection eliminated the background "roar and whine" to a point where I could hear more problems. The same riding a motorbike - put some ear plugs in and suddenly you can hear the engine and not the exhaust. So..... work with this guy, try different types of hearing protection when he is doing the task (and you may need to be with him listening too) and see if you can identify a type of hearing protection that will take out the exhaust and other noise and leave him able to hear the other bits.....But, be prepared for this to be a long road as he will need to be convinced that the outcome benefits the job as well as his well being Jim.

Hello mintcake ExDeeps is spot on with this, I have worked with racing engines for years and have always been able to wear hearing protection and carry out fault finding. You only have to look at F1 or Moto GP to see that all the crews wear hearing protection. If the correct hearing protection is worn it will only cut out the dangerous noises and your mechanic will be surprised how much easier it is to identify issues. Ear muffs might be better as if the mechanic has dirty hands/inspection gloves using ear plugs can give rise to ear infections if they are taking them out / putting them in regularly.