rddiamondd

I have a 1998 LX470 with 334K miles on the odometer. Recently I have began hearing a sporadic whistling noise during acceleration and cruising at highway speeds. Does not affect driveability of my LX but is extremely annoying and wondering if maybe its a sign of a current or potential engine or HVAC problem. It seems to be coming from behind the dash. Would appreciate a reply from anyone familiar with this particular issue? Thanks.