Notch Filter causes ringing?

I haven't noticed this before ... I'm working on another cleanup job - story telling recorded by someone else on a noisy system.  The notch filter is causing a significant ring.

The frequencies I'm notching are 60, 180, 300, 420, 540, 660, 780 ... there will be more.  It gets worse with narrower settings, but even narrow sounds bad enough that I don't think I'll go there.  Even just notching a couple of the frequencies will bring up the ring.

Any thoughts?

NR is also turning up more in the way of artifacts that I want - I'm going to try a few other things, but

That's the trouble with notch filters - if the rate of change of phase and amplitude is sufficient in the notch skirts, this can cause a ring just away from the frequency you are trying to attenuate. This is invariably worse if there is significant signal in them as well. This happens just the same in the real world too, and is part of the reason that people often don't like the sound of some brick-wall anti-alias filters.

In order to change the response rate very rapidly over a narrow range of frequency, effectively you can't damp the filter, so it has a high Q value. And like any system, electrical or mechanical, with a high Q value, it has a tendency to oscillate... as you probably realise!

NR with the freq graph set to process only narrow notches on the hum frequencies is working well.  Tedious to setup, but effective.  That along with a second, lighter NR pass to reduce broadband noise should clean things up adequately.

The author/reader was here for an initial editing session and she had decided she wanted to add one word to a sentence.  I recorded the one sentence and then had to try to make it match the rest of the session recorded with a different mic in a different room through different electronics ... ...

I ended up addiing some very short reverb (early reflections), EQ, some distortion, and I'll probably end up adding some very light hum and hiss to make it match the rest of the recording after I'm done with NR.   

It isn't bad - but it could have been a lot better.