If it was possible to make a legal move with the piece that you touch, it is my understanding that you have to make one.
touch move when in check

How can u not know that u are in check, I mean doesn't your opponent have to say check when it's check to make sure u see the check?
If you were in check and missed it - you were toast anyway, but yeah - if the touched piece can be used to block the check, you've got to play it.

I've played in tournaments with that rule and other ones without it.
I'm not sure if there is an official rule about it.

You must make a legal move with the touched piece if there is a legal move available -- If there is no legal move you are free to move any piece. By the way, touch your King first when you castle... if you touch the rook first and castle immediately without hesitation -- you will get a warning, (in the old days you were screwed and had to move the rook) -- if you hesitate at all, you must make an ordinary rook move. At any rate, why take chances, do what you are supposed to do and touch the King first when you castle.
I think your opponent HAS to tell you you're in check.........mhm
Nope. That's what makes bughouse fun.
Fide Laws of Chess
Article 9: Check
Section 3: Declaring a check is not obligatory.
Article 5: The moves of the pieces
Section 5.1C: If a player touches a rook and then his king, he may not castle with that rook and the situation will by governed by Articles 7.2 and 7.3 [Touched Piece rules].
(Or Article 4. Section 4.4 in 2005. I'm oooold school. Anyway, FIDE changes are never ending:))

masteryoda> I think your opponent HAS to tell you you're in check.........mhm
It's not required, and pointing out a check is even considered bad etiquette by many if both players have a reasonable tournament rating.

It's not required, and pointing out a check is even considered bad etiquette by many if both players have a reasonable tournament rating.
really? I didn't know that. Thanks for that piece of interesting info.

masteryoda> I think your opponent HAS to tell you you're in check.........mhm
It's not required, and pointing out a check is even considered bad etiquette by many if both players have a reasonable tournament rating.
Why's that bad etiquette?
I have played tourn chess for over 30 years now and yes if you touch a piece and it has a legal move then you must play it sorry if you have to use the queen to block the check As for announcing check, no it is not a rule and yes players with vast tourn. experience find it annoying. As d blackwell stated above if you are in check and don't realize it then you are probably in trouble anyway.
I have a question. When playing a tournament game, I was in check and didn't realize it. I touched my queen, with intent to take a pawn, and was told I had to use that piece to block the check. Did I have to use the queen or did I have the right to do a different move?