Well, it is a bit difficult to diagnose a noise on the net, but an engine noise commonly referred to as a "ping" is usually predetonation, is usually heard under throttle load and is usually caused by lower than required octane fuel for your compression ratio and/or ignition timing. Bad fuel is easy enough to detect (might want to do the fuel filter as previously suggested), altered compression ratio by carbon buildup on the pistons is, of course, more difficult, and timing is a couple-of-minute check. If a compression check indicates build up, a range colder spark plug may cure the problem...IF
that's the problem.
However, you indicated that the sound was also intermittent at lower RPMs and idle recently. This could mean an easy valve actuator ("lifter") change as Lono suggested. Or...you may have a leaking head gasket. Before you do dollars large, go to:
plug in the make/model of her Mitsubishi and see if there have been any recurring problems where a Technical Service Bulletin or recall has been issued for complaints similar to yours. Regards...Jack (P.S. The above address seems to be case-sensitive, so type in as I have done above).