Mine does the same thing. Emailed Garmin and they told me to make sure I had the latest software loaded. I did. Emailed them again and they gave me a return authorization number. 665 is on the way to Garmin.
Through reading some stuff, I determined that making sure the firmware/software was updated in the 660 was the first thing Garmin was going to try and say was the problem. There's a tread here started by Iski about problems with the 660, and it appears that some of the same issues occur in the 665. In a nutshell, sending the unit back to Garmin results in a hit or miss fix, as they send you a "refurbished" unit that may or may not have the same or different issues than the one you sent them in the first place. It seems that the consensus of opinions indicate it is better to go to your dealer with the problem and get a NEW unit. If I'm not mistaken, Iski went through 3 Garmin swaps and finally got a new one from his dealer. Garmin's customer service/technical department seems to be lacking...seems like they take the one's sent back to them, box them up, and send them back out as "refurbished" without really checking it out. I'm not surprised by that. When I get the NIM replaced, I'm going to see if I can get the 660 to repeat the Alvin and the Chipmunk thing...it seems to happen when the unit recalculates, or you do a detour. Once, while riding way out in BFE Alabama, going down into a canyon, mine emitted this LOUD, EAR PIERCING, steady tone. I attributed it to my location, but Iski had the same problem. Mine has not repeated that since. I could not make it stop with volume control...had to turn the unit off, wait a few seconds, then turn it back on. My half-assed educated guess is that all these issues are related to the pulse that the Roadtech verions of the Garmin 660/665 emits to interrupt the HK output to the speakers/headset, and it's f'd up, in addition to the NIM being an issue if it's not replaced with the new 11A version. Like RosanneRosannaDanna said..."It's always somethin' "