So being basically illiterate about cars except for keeping up fluids, tire pressure and being sensitive to weird noises or behaviors, I would like someone to explain what the exhaust manifold gaskets do, and how critical they are to the life of my car. I had noticed a "tapping" noise from my engine and not being satisfied with my still learning about Volvos Russian mechanic ("need valve job"), I went to my old trusted Volvo mechanic 150 miles away. His diagnosis is the replacement of those gaskets. Seems pretty major, with a major price tag too but I do trust his expertise. I'd just like a basic explanation for my own edification as I nurse this baby over 250k miles. And what else should I be asking my mechanic to check while he performs this bear of a job? Turbo air and oil cooler hoses? O2 sensor? (These questions came up as I perused your repair information) Thank you experts!