2005 Mazda 3 2.0 16 valve engine

2005 mazda 3 2.0 I lifted the hood and put my ear to the FRONT of the engine and heard constant tapping noise. My mechanic said the following:It does NOT have hydraulic lifters. All lifters have correct specs except four which have maximum specs. No metal shavings in bottom of oil pan. He went deep in the lower part of motor and did not find anything. Motor is clean.I put my ear on it and it sounds like it is coming from the second pipe from the left in the front of engine(it sounds loudest there). WHat do you think I should do?Ignore it or find someone else in miami, florida. Thank you. The car has power and runs well. I changed the oil but it still taps continuously. He said he would NOT touch the injector rails! Because he said changing the 4 lifters that have maximum specs or the injector rail replacing with a 2.3l from a mazda 6 he was not sure if positively would cause the noise to go away. He said all lifter specs are fine.
this proble is driving me insane and desperate!