I have a Lexus ES300. While idling and when shut off I get a loud pinging noise on the right side. The service department said it was the catalytic converter "cooling down" and that it was normal. I've heard the metal pinging of other vehicles cooling down and this is much louder. What in your opinion is the cause and solution?

I have a 1998 Ford Expedition XLT (Eddie Bauer) 5.4 engine with about 60k miles on it. I got a check engine light which turned out to be bank 0 and 1 reporting running "too lean". This was explained to me that there is not enough fuel, or too much air in the combustion. At the time, the car was not experiencing any odd behavior, but wanting to prevent any potential problems I decided to replace the fuel filter since I didn't know the last time it had been changed. After replacing the filter the car stalled for the first time. Ever since it has been off and on, having difficulties idling. It will drop from 1k RPM to 800 to 500 and sputter and sometimes stall. If I give some gas to increase the RPMs, it will sometimes help, but sometimes I can still hear it sputter (like it's running too lean). The next inexpensive (?) step was to replace the O2 sensors. Since I wasn't comfortable with doing that, I had a local shop replace them. I also had them clean the fuel injectors and lines. The problem seemed to go away after a time (we assumed the computer had reset), but then came back. From what I've seen online it seems that the next step is to check the IAC or the TPS sensors. I saw one post that chastized the "random replacement of parts" in an attempt to diagnose the problem. Is there really a way to test these sensors without replacing them? With a diagnostic code "Running Too Lean" which could be cause from 10 different sources including bad sensors, what is really to be done? I'd love to hear from someone who has had a similar problem and was able to fix it. This is just one more reason why I don't like Ford's. My previous problem was a "misfire" on plug #4. Ford decided it would be a good idea to require dropping the engine before the back plugs can be replaced. Good grief - on a F-150 body, the most common service duty truck in the US? What design wizard came up with that? Thanks for listening to my rant. I'll post again once it's been fixed.