I go into a mechanics and ask them whats wrong with my car. I give them a minimum of information and let them go from there.
If what they are suggesting makes sense to me, then they get the job. I may be no expert but I know enough to know if something seems right or not.
If I dont understand what they are telling me, or I know little about cars, I think I would go to more than one mechanic, telling them as little as possible, and seeing what they all come up with. If they all agree, then you can be sure its probably correct.
Go into a workshop with a noisy tappet and tell him your engine is dying you need a new one, he will quote and fit a new one. Go in and say what the hell is that noise, and he will tell you its a tappet and it will cost X dollars to fix.
Its just stupid to go and ask a pro whats wrong with your car whilst telling him. If you already know, dont ask. If you're sure its a tappet.. just tell him to fix the tappets, dont ask him anything. If you arent sure, shut up and ask him what he thinks the noise is.
If you genuinely have any mechanical knowledge, you'll be able to figure out if his opinion is plausible.