<<oops -- hit the send button way too quickly and wanted to add this
too>>

Just to clarify:
Is it truly revving that high? 5000 is something your engine does not
normally see very much -- if it does rev that high for no reason, you
could do damage to the engine. Don't slam it in gear at that speed, as
that will also do damage to your transmission.
To make sure, the engine speed (thus noise) increases, right? If it
does not, you may have some sort of problem with your tachometer.

I don't have my nissan book handy, but most cars contain a load sensor,
sensing electrical load changes. When a load changes, the engine speed
picks up slightly under nrmal conditions. If that sensor is bad, your
engine will do what you describe.
Tail brake lights cause more current to be required - that's why I
asked if it also happens when you turn the headlights on.

Remco