2000 Suzuki Grand Vitara Engine noise

I have a 2000 Suzuki Grand Vitara with 83000 miles. I notice a significant drop of fuel economy from 8km/L to as low as 5km/L. Its been two years since I replaced the spark plugs (iridium OEM). Recently, I have a tune-up and the mechanic replaced the spark plugs(iridium OEM), fuel filter, clean the K&N air filter and requested to replace the two front oxygen sensors with NGK oxygen sensors (I suspected that the drop of fuel economy is cause by the O2 sensors). The fuel economy improves a little bit but no longer reaching 8km/L. But prior to the tune-up, I notice that when I cold start the engine, I hear this noise coming below but as the engine comes to its normal temperature the noise is gone. It never happens before, the engine is running smooth. I mean, I can't even say the engine is running. I'm just wondering where the noise if coming from and the engine is running like a diesel engine maybe from those loose valve lifters. I use an OBD II scanner to check if all components are running good, and the scanner flash a "0" reading. So, I assumed everything is okay. I also ask the mechanic to replace the PCV but they told me, my SUV has no PCV? Questions: 1) Where is the nose coming from during cold starting? 2) Is there a PCV on a 2000 Suzuki Grand Vitara? 3) Are the new front O2 sensors improve fuel economy? 4) How can I reduce the noise from the valve lifters? Thank you.