I've got intermittent chirping (only while pedalling) at probably the rear derailleur and I may have to replace the pulleys. However, I'm doubting my diagnosis since I replaced the original eight year old pulleys only three years ago (Shimano RD-M593).

try dripping some oil on the bearings.
it's part of basic tune ups in this shop.

Simplest stuff first. Put the bike on a work stand, grab the right pedal, and pedal backward slowly while listening to the chain as it runs through the rear derailleur. If the chirp happens once every rotation of the pulleys, the chirp is originating there.

If the chirp happens about once every three or so revolutions of the cranks, it's the chain that's chirping.

Lubrication is needed in either case.

Definitely oil the pulleys, especially if you notice it while shifting.

Check the chain ring bolts cassette cassette lock ring. Though you say it only happens while pedaling, may as well check the disc rotor bolts or center lock if you are disk brake. Or if rim brakes, is the rim out of true and scrubbing a brake pad.

For a different possibility... We've seen/heard a chirping from hub axle seals. Some hubs (mostly moderate cost ones) have a slip on "rubber" seal on the LH side of the hub. These seals can rub on the hub shell just so and emit a faint noise, more easily heard when pedaling as the ratchet sounds can overwhelm the seal chirp. A small drop of oil between the seal and hub shell fixes this. Andy.

“Only when pedaling” Have you checked to see if you still hear it when pedaling out of the saddle? If it doesn’t, then I would also check the seat clamp (and maybe the seat post too). Take the seat completely off the clamp, check for, and clean the seat rails and clamp. A small bit of corrosion in there can cause the connection to become ever-so-slightly loose. As your body slightly weight shifts side-to-side when pedaling, it makes the seat move just enough to produce a ‘noise’ from there. If it makes the noise when you’re pedaling out of the saddle…check the headset, handlebar connections.

Dan
​​​​​​​

I've de-greased, WD40'd, finished off with washing-up liquid and followed up with lubrication. I'll know when I get out on a test cycle if was down to gunge.

In the meantime, some people wondered if the derailleur cage could be bent - to my untrained eye it looks straight.

Anyway, if I get chirping, it'll be easier to replace the pulleys - they're not expensive.

And if it doesn't happen at all on the stand, but only when riding, it could be the pedals, cleats, or saddle.

I ride Speedplay X pedals and when the clear springs are getting worn where they contact the pedal axle they can squeak. It took a lot of miles, miss tried paths (pulley wheels and chains were cleaned and lubed a number of times) and frustration each time I thought I had found the cause. It was placing a dab of grease on the pedal axle where the cleat spring can touch that proved where the noise was coming from.

I've heard/seen seat posts creak, BB and crank bolts do the same. But these types of causes generally have a short and "tight" noise and not a longer "drawn out" one like a squeal. But one person's squeak is another's creak. Andy.

My plan worked - the bird has flown. Thank you all for your advice which will hold me in good stead if Plan A doesn't work in the future.

Best way to thank us is to tell us what the cause of the chirping turned out to be.

I wish I knew. After the afore-mentioned de-grease, WD40-ise, final clean with washing-up liquid in warm water and follow-up with lubrication - I took her for a fast 7 mile trip. Not a single chirp.