Frame not suitable for 70" fixed gear?

Huge pedalling forces aren't solely the preserve of fixed gear riding.

Either the frame has a flaw and is approaching failure (maybe the least likely option), or it's something else. I've heard sounds that were ***definitely ***coming from the BB area, but turned out to be:

• cleats
  
• external rubber seals on pedal axles
  
• saddle rails
  
• burrs on end of seatpost against seat-tube
  
• bar/stem interface
  
• cable/down-tube stop interface (okay, not applicable here)
  
• loose chainring bolts
  
• riding partners
  
• blah