What’s happening here?

[OneChordTrick]

Just put some new strings* on, well it’s almost solstice , and I’m getting an odd resonance. When I play a D on the A string, 5th fret, I get a resonant note I think a D but higher on the G string. I’ve isolated it to that string by muting the others.

It’s a new brand of strings to me and steel rather than phosphor bronze if that makes a difference.

* they were a cheap eBay special £3 rather than my usual set at about £10. For reference a set of EJ74s is about £8 over here so I didn’t have high expectations. In fact I’d almost decided not to like them before I hit the buy button but it seemed too good to be true.

So is this my innate predjuice looking for a flaw, due to the switch to a different material, slightly different gauges or just one of those things when you switch brands?

[Tezzerh]

Or maybe a case of you get what you pay for? It was worth £3, though!

[OneChordTrick]

That’s the most likely explanation

[Tom Wright]

Entirely normal and was happening before you put on the new strings. The different tonal balance of the steel-wound is apparently making that natural sympathetic resonance more audible. There are conversations here about when to damp, or not, for musical purposes, and some instruments (sitar, viola da gamba) have extra strings whose role is to sound sympathetically for a built-in reverb.

[Ray(T)]

New strings of whatever brand are going to be more lively than the old ones. I’d start by making sure you’ve damped out the strings between the bridge and tailpiece and the nut and tuners and see if the problem is still there and really is down to sympathetic vibrations.

You’ll find that it happens on most mandolins, no matter how bad or good the individual instrument is. There’s nothing wrong with it and such vibrations and overtones all contribute to the sound of an instrument. If they were totally absent, an instrument would sound very boring!

[Folkmusician.com]

Agree, that the fresh strings are probably just bringing out a characteristic of your Mando.