My '05 titan had that same sonar probolem today....for the record it was 19 degrees outside and there was some freezing rain last night. I know this sounds like a no brainer of a probolem of simply wiping off the sensors, but i swear the sensors were clean when i checked them and i wiped them off just in case and the probolem still continued. Later on that today it got to about 33 degrees and the solid beep went away. I will go to the dealership with this TSB info later.
http://www.infinitihelp.com/Ownershi.../NTB04-102.htm