Hi scotty, I have a 08 mazda 3, 2L with 156 000k i have been hearing a sound that i can not find the culprit of, it started maybe a month ago out of nowhere. this appeared before oil change, so i tried kleen flo engine flush in crank case for 5min @ 2000rpm in neutral, changed oil (&filt) with recommended weight castrol gtx synth blend and a recommended blend of napa synthetic stabilizer, i cleaned the maf sensor, changed air filter and cleaned throttle plate but it was pretty grimy so i maybe suspect the valves are just as bad but i havent managed to use valve cleaner, and no engine light. besides that everything seems to be solid, performs and sounds good, besides some alternator pulley play but is not the direct source of noise. i cannot find recreations online of the sound my engine is making. When i start it cold it sounds fine, until it reaches below to operating temp, becomes noticable @750-550 idle rpm, mostly noticable inside the vehice when parked but can be barely heard when inspecting the engine up close. It is not a metallic sound, but it is almost kind of a hallow click without a metallic noise if that makes any sense, it gets worse when i am parked and have the clutch in with it in first but after taking off it goes away with added rpm, it still doesnt sound metallic at any point it just gets louder and it is at fast pace when it starts but doesnt get much faster sounding, once i get going it cant be heard over about 1400 rpm. In my manual it says a periodic valve lash adjustment is suggested around 110000k but only when it can be noticabley heard, im just not sure if its the noise i am hearing or not, i am the 2nd owner of the car so i have no idea what exactly has been done, also only 19 so its hard to go to the mechanic and pay a few hundred for something that might not be the problem, i have also been using napa fuel additive to help clean everything out further & getting over 500k/tank. thank you so much for taking time out of your day to read this Mr. Kilmer any help is appreciated!

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do this video to find the source https://www.youtube.com/watch?v=pqh1oH8mEqY&t=115s