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Hello experts, I own a Toyota IST (2002) model. This model has 2NZ-FE VVTi engine (1300cc), driven about 85,000 KMs. Couple of weeks back, it started making an occasional knocking-like noise and I realized this only happens between gear shifts (automatic shift), specially when i drive slow. To be specific, if I drive slow in traffic, when i slightly accelerate and when it is just about to shift from 2nd gear to 3rd, it starts with this rattling noise and after a couple of seconds it goes away. I never get this at idle or even if i rev up the engine and only happens when driven. I wasn't too sure if the noise was coming out from the timing chain compartment, therefore I took my car to my regular mechanic which he overruled the timing chain after checking, suspected with knocking and advised me to try an EFI tune up which didn't make any difference. Then he suspected the knock sensor and/or the coolant temperature sensor to be failing intermittently, which i replaced them, still no luck. I also replaced the mass airflow sensor. I must say that it never came up with a "check engine" light and multiple scanning reports showed that tuning was right on and didn't indicate anything wrong. After consulting many repair folks with no luck (which most of them confirmed that it was knocking yet unable find a proper cause), I finally decided to try my last resort - took it to Toyota for a full inspection of the problem. They opened up the hood, inspected the noise, had a test drive. Their diagnosis was that it is caused by a worn piston rings or a valve problem. They also did a compression test and confirmed that cylinder 2 has less compression (around 8:0) which the other 3 cylinders have the expected 10:5 ratio. I don't consider myself a geek in this field yet I am not a dummy either. I do have some knowledge of how it works and what could cause what. Therefore, though my only lead is to go by Toyota's diagnosis, I would like to seek your expertise and opinions with regard to this before I go for an overhaul of the engine. Could a worn piston ring or values cause an ignition knock or a similar noise? I am truly worried if I would end with bigger issues if I don't fix it right now itself. It would be great to know if I have any other option and I am willing to take my car to any expert in this area. Greatly appreciate your thoughts... Note: I do not have any symptoms of oil burn and do not get any blue smoke coming out of exhaust. I also get good gas mileage. Didn't notice a timing miss either... Thank you! -Isuru