Test : When this happens again .. Turn key on but do not start engine, let all of the tones stop sounding and roll up windows so the environment is as quiet as possible .. Now depress brake pedal and listen closely for a click from the shifter interlock solenoid... Each time the brake pedal is slightly depressed you should hear an audible click sound from the shifter area if the interlock is working properly .. Post results.
Hint : You can try this test while it IS working properly so you'll know what to listen for when problem arises again.
Done that more than once. This is the third year in a row that this has happened and like I mentioned it is only in the VERY hot months of July and Aug, sometimes I put up the shade on the front window and leave a window cracked so it doesn't get hot inside ( can get as hot as 140 without it being up) and it seems ok. I have replaced the switch that runs under the dash along the brake pedal thought that was the problem, seemed ok after that but it started up again last year and now again. Is there a switch sensor or something that is affected by the high heat temps in the ignition area or in the shifter knob? I have done the shift placement several times when it is working properly and the sound is faint but I can hear it. One thing I noticed was that the steering wheel lock, when you turn it on doesn't seem engaged, it has a free turn with no resistance, is that how it should be. Have read all I can in my manuel and auto books but nothing addressed it till I got on your site. Thank you for the input.. saw the info on the gear position sensor, that helps..
You still need to do the test I recommended to determine if the solenoid is working or not ... However here's my guess - the shift lock mechanism has been contaminated by soda, coffee etc spilled *into the shifter cavity'* .. Hot temperatures turns this into a sticky syrup that impedes shifter release .. Also possible intermittent shift-lock solenoid failure. .. Will need to be tested while it's acting up of course. (Liberally spraying with WD-40 is an effective way to clean ALL the shifter components) Good luck with it!
Bingo! Thank you forgot that that happened the year after we replaced the switch on the brake pedal rod, that may have been the solenoid switch because it worked great after that, BUT in the fall we had an accidental spill of coffee in the shifter area as you said. Then last year in the summer this problem started, happened a couple of times, stopped as it cooled off and then again this year it just started., will try the WD40 as you suggested and let you know. Since it is intermittent it will be hard to tell but it sure sounds like the problem as I know the spill occurred and it was sticky..
Concentrate WD-40 spray (with red straw attached) into the button on shifter handle AND down into shifter cavity .. Shift transmission back and forth a few times then repeat... I use compressed air to help the process but I have that readily available at the shop, that 'may' not be necessary but should be considered. Good luck with it!