Sounds like the upstairs wiring circuit is the problem. Most likely places to look are bad telephone (swap phones) connections, line running too close to electric lines, or transformers, rodent activity, cut-thru by staples, etc. You could disconnect the dsl feed just to chk for noise from that, but I doubt that is the prob. I either solder-gun all connections, or use pliers-activated gel splice connectors (Google for Images of telephone gel connectors ). Either of these methods makes a quieter connection than screw posts or terminal strips. If the noise is fixed by tightening the connections, then just substitute an improved connector. If the connections are not the problem, and you have checked for staple cut thrus, then rewire, but don't use a stapler - that is asking for trouble, unless you use one of the expensive ones the tel. companies use.. I use hand hammered insulated stables suited to the guage of the wire. The final test of the existing wire would be to just run an unattached wire thru the house up to the phone, and try it before you rewire the whole route.
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