Replaced chain. Still making the same sound under accelleration.
 

21sp Dahon Espresso. 2 years of hardcore commuting service after buying in used/new condition. Began to make a brrrrp sort of noise and vibration I could feel through the pedals on the "power phase" of my downstroke (rat trap pedals, no clips.) Only under hard acceleration, and as far as I can tell, it's doing it with no difference in sound in every gear. At least in 2 and 3 up front and 3,4 and 5 in the back.

I figured it was most likely a worn chain that was now stretching under load (cheap KMC) and replaced it.

Absolutely no improvement. I would think that if the chainrings were worn. than a new chain would cause at least a different sound and feel running over worn teeth. It is a very cheap looking triple, though.

I have never had any trouble with skipping, and I ride hard over rough urban roads...

When I've had bottom bracket issues before on another bike, the sound was crunchy regardless of the power I was putting in.

So what gives? Is it time for a BB overhaul? Did I screw up the freewheel when I replaced a driveside spoke last week? It did not make the sound after putting the wheel back together.

I don't have the tools or parts to do the BB...

Any ideas?

Sort of hard to tell from the description... but I had a similar 'rough' sound on my road bike when the chainrings were worn... I replaced the cassette and chain and it was louder than ever. Replacing the chainring made it good as new.

you need to isolate where the sound is coming from. Not likely that it will make the same noise on the stand but if you haven't tried that I would. When the chain was off did it seem that the rear hub, freewheel and bottom bracket all spun smoothly? How about your RD pulleys?

Rear derailleur could be slightly out of adjustment.

+1 for the pulleys, if these are pointed, the need to be replaced, if you have done 2 years of commuting on it, could expect to have to replace the whole drive train (cassette, chain, pulleys, cables & possibly chainrings after that long, if in daily use, dependant on conditions traveled in, periodic maintenance carried out and distance traveled.

derailler hanger, Bottom Bracket & freewheel/cassette hub should be checked as well

consider the rest of the drivetrain, cogs and chainrings wore to suit the 'stretched' ie worn chain,
so Now also need replacement.

Check bb. Check pedals.

+1 but check pedals first, also your cassette for wear.

I've never heard a "Brrrrp" sound from a bike, but it it was a "Blaaaft" sound, I could tell you exactly what the problem is.

Blaaaaft usually only happens when I radically change my diet.

Twelve links on the chain were not quite 12 and 1/8", So I was hoping that I'd gotten to the chain replacement before doing damage to the rest of the drivetrain.

I rode home from work after posting and suspect the chain rings -- they're really cheap, and the middle is making the noise a little more than the big ring, but it's hard to tell b/c I don't have the same strength in the PM as in the AM. (no coffee!)

I'd actually like to pull the (low miles) triple off of my jamis, so I have one that I could use.

My pedals are from a 1973 Atala and have probably never gotten new grease-- I'll service those once I figure out how to open them up.

So if I replace the chain rings and BB, I might as well do the freewheel. Lame. Sounds like a full drive train overhaul is in order. And I just blew the bike budget on an xtracycle. Oh well.

I cleaned the pulleys thoroughly when the chain was off. Everything else seemed to be spinning smoothly. I didn't give the cranks a good spin, though. Next time I'll know to do that while the chain is off. I was only spending time cleaning things. I didn't even realize the problem was still there until I hit the road this morning.

I was paying close attention to the pulleys on the ride to see if they were the problem, but I don't think so.