TDM410P noise on FXS ports

I am experiencing a noise problem on the FXS ports of the TDM410P card in my system.
The FXS ports are normally connected to a GPT PKS-2 PBX, but the noise is present
even with just dumb analogue handsets connected directly to the FXS ports. The noise
is only audible on the FXS lines; the other party in calls does not hear any noise.
It does not make any difference whether the other party is connected via SIP, or the
FXO port on the TDM410P card.

I have so far tried disabling echo cancellation and providing external 12V supply to
the TDM410P card from a battery. Neither resulted in any change to the noise.

For system details, see http://www.laczik.org/download/tdm410p_noise/voip_system_details....

I have recorded a few sound samples, both with asterisk running and calling into a conference,
and without asterisk running:
# dahdi_monitor 2 -m -T tx_conf.raw -R rx_conf.raw -l 200000
# ; call conference extension
# sox -r 8000 -s -b 16 tx_conf.raw tx_conf.wav ; sox -r 8000 -s -b 16 rx_conf.raw rx_conf.wav
# amportal stop
# dahdi_monitor 2 -m -T tx_2.raw -R rx_2.raw -l 100000
# sox -r 8000 -s -b 16 tx_2.raw tx_2.wav ; sox -r 8000 -s -b 16 rx_2.raw rx_2.wav
# dahdi_monitor 3 -m -T tx_3.raw -R rx_3.raw -l 100000
# sox -r 8000 -s -b 16 tx_3.raw tx_3.wav ; sox -r 8000 -s -b 16 rx_3.raw rx_3.wav
# dahdi_monitor 4 -m -T tx_4.raw -R rx_4.raw -l 100000
# sox -r 8000 -s -b 16 tx_4.raw tx_4.wav ; sox -r 8000 -s -b 16 rx_4.raw rx_4.wav
# amportal start
The wav sound files are available at http://www.laczik.org/download/tdm410p_noise/
With dahdi stopped (/etc/init.d/dahdi stop) the noise disappears, but when dahdi is
restarted the noise reappears again.

The noise is much more pronounced through the handsets than in the recordings and
tends to interfere with the voice in the calls, resulting in complaints from the users of
the system.

Any suggestions for the cause of the noise and a way to eliminate it?

Thanks,

John