2004 cayenne S PSM issues

[jrgcom]

My 2004 cayenne S with 188,000 (I am original owner), has a flashing PSM on dash when rpm is above 4000 or above 60 mph, in stop and go traffic, I noticed if I speed up to about 20, slack off throttle, there is a couple of clunks coming from the rear of car (does not appear to be shifting, not an unusual hard shift). Also having a code P0491 which is secondary air pump. Are these related?

[lewisweller]

20 hours ago, jrgcom said:

My 2004 cayenne S with 188,000 (I am original owner), has a flashing PSM on dash when rpm is above 4000 or above 60 mph, in stop and go traffic, I noticed if I speed up to about 20, slack off throttle, there is a couple of clunks coming from the rear of car (does not appear to be shifting, not an unusual hard shift). Also having a code P0491 which is secondary air pump. Are these related?

The code I not related in sure. The clunks may or may not be related to the psm warning but speed sensor is likely as you mentioned as specific speed, and no way 4000rpm is 6th gear at 60mph. So non rpm related would indicate its Road speed related which leads me to speed sensor first. What's the code related to the psm? 

Also don't rule out unmatched tyre pressure (rolling radius difference), different tyre types, tread pattern and or tread wear excessively different on same axle. 

You could jack up safely and spin each rear wheel listen for dragging noise, remove and clean speed sensors. 

Need more info to continue after that.