A talking lute? Surely you must be mad, old man

[DMLoeffe]

Rhovandir, Morko, Dunner and Isabel, please look away

Recently one of my players found a magic lute, which talked to him a bit, telling him to pick it up, etc. But the thing is, the lute doesn't actually talk to him, it's all in his head. The character is an old man who has gone just a bit insane during his time trapped in the Astral Sea, and I think it would be funny if it turns out he's just imagining the voice. My problem is that I don't quite know how to make this known to the player. I don't just want to tell him outright, but it shouldn't be completely non-obvious either, as the campaign is wrapping up in a few sessions. Any advice on how I can roleplay this voice in a way that indicates that it's just a voice in his head, or otherwise drop some hints to this fact?

Thanks!

[The Glyphstone]

Have it know things or talk about things only hes seen or done, things it shouldnt be able to know about.

[Bad Wolf]

Works in silence effects and other places where magic is suppressed?

[Lvl 2 Expert]

Secretly inform the other players that they don't hear anything, but suggest they try not to call him crazy?

[Deophaun]

You don't have to engineer anything to make it known to him. Just make sure that you operate within the premise and don't allow the other PCs to act as if they were hearing the lute. The player should only discover that his character is touched on his own initiative by trying to suss out what the deal is with the lute.

[Lvl 2 Expert]

By the way, do you know this player well enough to know he will not start yelling about you taking over his character by making him crazy? Just checking...