When engine is first started cold REALLY LOUD noise under vavle cover on pass side of motor then goes away..my mechanic pulled valve cover and everything looks fine nothing loose.. he thinks a lifter is not pumping up properly..can they be changed without taking off head,and how hard is it...engine has 151,000 miles...should i change all of them or just the one thats not working properly...dont want to keep driving and break something...thanks