Noise when riding
 

Today I noticed a strange "clicking" sound when riding, so I stopped to check the bike. I tried spinning both wheels but couldn't hear anything, so I continued riding (with the noise). It sounds as if the cyclocomputer magnet is touching the fork or something, but I checked for that. What could it be? I can only hear the noise when riding.

Try this:

http://sheldonbrown.com/creaks.html

If not the obvious chain issues, check the pedals, crank arms, and bottom bracket. In that order, make sure there is no lateral play in the crank arms (perpendicular movement to the frame).

You many find the following links helpful in tracking it down:

http://sheldonbrown.com/creaks.html

AND

http://www.parktool.com/repair/readhowto.asp?id=123

It's usually just a matter of patiently removing, cleaning,
re-lubing, and re-torquing fasteners until it quiets down.

I thought it might be the drivetrain, but the sound continued even when not pedalling.

From http://sheldonbrown.com/creaks.html, it seems most likely one of my wheels (possibly the rear one) has a (some) loose spokes. How would I go about tightening them? Do I need a special tool?

If you have never trued and tensioned wheels before, I suggest you get it checked by a reputable bike-shop (best bet is a cluttered one that's been in business for years). And let them handle the truing/tensioning for you this time.

Then take the time to learn how to do this yourself. Get a junker wheel to practice on and a spoke-wrench. There's much to read and learn available to teach you the physics & principals involved. It tends to be both a science and art-form as well.

Good luck!

I've managed to tighten some of the spokes that felt loose. However, I still kept hearing noises. So I checked the wheel again, and a few spokes were loose. That kept going for a while, until the noise reduced significantly (I heard it maybe once or twice during my ride). Recently, it started again. So I checked both wheels, and it appears the front wheel has two bent spokes. What am I supposed to do now?

^ Be careful, you cannot just randomly tighten and loosen spokes. They all work in tandem to give strength to the wheel. Do what Panther said and take it to a good LBS. Bent spokes, depending on how bent, either must be replaced or should be replaced.

I took it to my LBS, and they retensioned the wheel. However, the mechanic said it might still make some noise because the spokes are "adjusting" themselves to their new tension. Is this correct? I rode 8km back from the LBS and I can still hear noises.

Not really. One, I've never had noises last that long - 8K is a long distance. Two, a good job includes manual stressing/de-stressing of the spokes that minimizes spoke self readjustment.

Notice Panther said *reputable* bike shop. Some shops will just do what you did and tighten and stuff by hand, which can be fine with an old experienced hand. But in all likelihood they need to check the tension of each spoke and they may not have.

My Cateye Enduro 8 computer actually does make a faint ticking sound where the magnet and sensor pass each other (they don't touch). You don't hear the noise when the magnet-mounted wheel spins, while not riding?