Over the past week or two, my bike has developed an annoying squeak when the left pedal is at the lowest position. The squeak sounds like a metal spring flexing.

I've tightened the left pedal crank, cleaned and lubricated the pedal and tightened the bolt attaching the saddle to the seat post, but the sound continues.

When I'm standing and out of the saddle, the sound goes away.

I replaced the saddle around 1,000 kilometres ago and at that time, I also had the bike tuned up.

Any suggestions as to how I can silence this noise?

Thanks for your suggestions.

Could be coming from the BB... if you have the tools, you could try disassembling and reassembling it with a bit of grease.

There's your first clue.

Before you go crazy with cranks and BB, remove and replace the saddle with any other if you have something laying around. If it cures or changes the sound you know the cause for sure. Modern plastic saddles often develop squeaks where the top is attached to the rails. Depending on your particular saddle the solution could be injecting glue or lubricant into the joint. BTW- some saddle squeaks are almost uncurable, and the only solution will be to let it do it's squeaking in the scrap bin.

Check the cleat on your shoe, too.

+1

I chased a squeak that I swore was coming from a SPD pedal after every rain storm.
-swapped pedals (after attempting multiple lubrication types and adjustment)
-swapped shoes !

Finally after 1 year I slightly tighted my brooks saddle. E-gads the noise was back. I am convinced it was the saddle the whole time.

There's one bolt on the saddle, where the rails clamp to the seat post. I've got this tightened. Is there anything else I need to tighten or is there something I need to lube? The saddle is comfortable and I'd prefer to keep using it instead of swapping it out.