Air Carrier Captain reported the Radio Altimeter display on their Heads Up Display blinked and they received an unstable ILS signal while on approach.

Narrative

We were visual with the runway and flying the ILS Z 16R; after Glideslope intercept; the Localizer course in the HUD started to flutter from side to side but never lost signal. At approximately 200 ft. the Radio altimeter in the HUD also blinked a few times; but never lost signal. All altitude call outs were normal; 100; 50; 40; 30 20 10 ft. I did not write up anything in the AML; but did contact Tower to relay what we experienced to see if they were having any issues with the ILS. At no time did I feel the approach was unstable or not safe because we were VFR with the runway. However; given the 5G issues; If we were flying a CAT III approach I would have executed a GO AROUND because the ILS signal did appear slightly unstable. This is a hard event to describe because no instrument failures or equipment failures occurred. It just wasn't normal! The ILS signal appeared slightly unstable; but reliable. Again we were VFR so continued the approach. The fluttering from side to side was no more then 2 degrees. A better word to describe the fluttering maybe vibrating from side to side in a rapid motion all the time staying within the localizer course. The blinking of the Radio Altimeter was also rapid and less then a second between and only happened a couple of times. First Officer as far as I can recall did not notice anything on the Dual Cue or the PFD. Only in the HUD. It just wasn't normal. No real suggestions because I cannot point to an instrument or signal failure. It just wasn't normal. But at no time was it unsafe.