I - and others - observe that when a musical performance ends, the echo of the last chord appears to rise in pitch by up to a quarter tone while the echo decays. This effect appears to be independent of the type of performance - orchestral or choral. I am tempted to assume that this is related to what is known in electro-acoustic circles as Space Echo, but I cannot see why this should occur.
Just found this:
Hartmann, W M. “The effect of amplitude envelope on the pitch of sine wave tones.” The Journal of the Acoustical Society of America vol. 63,4 (1978): 1105-13. doi:10.1121/1.381818.
Psychophysical experiments show that the pitch of a short sine wave tone depends upon the amplitude envelope of the tone. Subjects find that the pitch of an exponentially decaying tone (1dB/ms) is higher than the pitch of a (20‐ms) rectangularly gated tone of equal frequency. The percentage difference in frequency required to produce equal pitches with the two envelopes depends upon frequency $f_0$: 2.6% at $f_0$=412 Hz, 1.4% at $f_0$=825 Hz, 1% at $f_0$=1650 Hz, and 0.7% at $f_0$=3300 Hz. The pitch change is insensitive to the relative intensities of the two tones. The spectra of tones with the two different envelopes suggest no obvious explanation for the pitch change. However, the weighted time‐varying spectra for tones with two different envelopes evolve differently with time. Alternatively the pitch change can be derived from a modified version of the auditory phase theory of Huggins.
So it's a known psycho-acoustic phenomenon — we perceive the pitch of a decaying sound to be higher than that of a constant sound.
I experience this too, and it's frustrating as I cannot think of any physical explanation. As far as I can see, unless there is a doppler effect due to a moving surface, the frequency must remain constant as it dies away.
My conclusion is that it is a phsychological effect, ie. the pitch we perceive is affected by the volume. I don't know if this has been tested scientifically but it would be a simple test. Just play a note from an oscillator, and suddenly decay the amplitude while keeping the frequency the same. I believe we would experience a rise in pitch. Even more interesting would be to play a decaying note in reverse to see if the pitch goes down. It would also be interesting to know if this is universally experienced.