Since your telephone RFI is new, the ADSL modem is probably rectifying RF.
An ADSL filter (I worked on this stuff at my last employer) is usually built into the modem and into the telco equipment. A series capacitor blocks audio from coupling into ADSL circuitry. However, protective diodes are included to deal with ring voltages and transients, and these WILL rectify RF. Since their forward voltages are not well matched, the rectified RF is likely to show up as a differential output at audio frequencies.
Since ADSL uses spectrum up to 1.1 MHz, a low-pass filter before your ADSL modem, with a cut-off around 2 MHz, might well reduce this effect.
There is another possiblity, which is that RF is entering the modem by way of the mains conductors. I observed such a problem on an early ADSL modem myself, but as it was not our equipment, I could do little but advise the manufacturer. However, a sturdy common-mode powerline filter dealt with it.
Cortland