The way our ears work, what frequency we actually hear shifts slightly with intensity.  There is a great book on acoustics by F. Alton Everest that goes into this a bit in one of the earlier chapters.  I am still reading through all of it, but it is an interesting phenomenon.  Pitch is in actuality a subjective form of Frequency, not objective, and this is reflected in this phenomenon.

> The level of sound affects the perception of pitch. For low frequencies, the pitch goes down as the level of sound is increased. At high frequencies, the reverse takes place—the pitch increases with sound level.

> Everest, F. Alton (2000-09-22). Master Handbook of Acoustics (Kindle Locations 1553-1555). McGraw-Hill. Kindle Edition.

There is a nice experiment involving two oscillators they use to demonstrate this.  The book is well worth the money if you can get it and read English natively(Not sure what translations exist or how good they are).  I am still working my way through it but it will likely take me two or three times through before I fully understand most of it.

            Seablade