low idle in drive only

What is your question?

does not idle high no matter how cold it is. starts up and idles at around 700. then after driving a little idle drops to 500 rpms. put in park idles fine. turn on defrost and idles fine. also when started after sitting, sounds like valves chattering or no oil in top of engine. only last about a second then sounds fine. after that starts up fine unless it sits for awhile.