Mystery symptom
 

I've been riding for many years now, and do most of my own work. I have encountered a symptom that has got me baffled and was hopoing someone here could help out.

the sound is a rythmic scritch scritch scritch sound. It goes faster with increased speed of the wheel. I only hear it when pedalling, not gliding. I have to be pedalling fairly firmly, just spinning the pedals doesn't produce the sound. It gets worse as I ride and I often won't hear it at all for 5 miles or so. The speed of the noise does no alter with pedal speed differences. The speed of the wheel seems to be primary.

I have eliminated anything rubbing of the wheel as a culprit. I opened up the brakes, so it's not the brake pads.

Thanks for your input. I hope someone here can tell me what this is.

Forgot to mention, the sound is coming form my back wheel.

It sounds like this is only happening when a certain amount of flex is present. Try using some WD 40 at the points where the spokes cross as these stick together and can cause noise.
Again I'd check for clearance if you use mudguards and check that the wheel is sitting dead centre in the chain stays. It's difficult when on the bike to ascertain just where the noise is coming from, so perhaps if someone can ride alongside and listen it would help.

Are your chainrings tight? I once was stumped by what sounds like a similar problem until I found loose chainrings.

I know you checked for things scraping the rear wheel... but what about the overflow cable from your front derailleur?

It could be the saddle.

My experience has been George may have it. If the noise goes away when standing..try greasing saddle rails and checking the saddle structure. And make sure the rear QR isn't shifting slightly.

on top of the suggestions made here, a couple of others I have experienced
1. Shoelaces hitting something, at high cadence, they flow out more
2. Spoke to nipple interface or nipple to rim interface, sometimes when pedalling hard, more force is put on the rear wheel.

Do us all a favor and report what it is once you find it
Thanks

Do you have a spoke protector on the rear wheel?

Could it be a bad bearing in the rear hub, a flat spot? It may take a certain amount of force from rider weight and firm pedalling to create enough friction to make the sound. Increased heat from friction would cause it to expand and get worse as you ride. The bearing could occasionally rotate off the flat spot, making the sound go away until it spins back around to the flat spot.

So hard to guess without hearing it. Maybe have someone else ride along side and listen for exactly where the sound is coming from.