Strange noise when engine stops - advice for buying bike

[Solon]

   I finally managed start work on my bike. Nothing wrong with fuel tap, or spark plugs, or even ignition coils, just the coil connectors were kind of loose and no longer kept contact between wires and coils, that's where the misfiring came from. Cleaned the carbs and now the engine accelerates instantly and smoothly. Now I'm just left with changing oil, filters,  maybe steering head bearings and a fork service, and possibly front brake discs. Nothing much, comparing to my initial fears, and then the bike will be good to go. Or will it?

   After stripping the bike I found that the chassis looks pretty odd, like it's been beat up in a serious accident, and that really gives me something to worry about. That ugly welding and rippled main bar look repaired, not factory made. On the other hand, the bike certainly steers well, doesn't pull left or right, and the ripples look very symmetric.
   And, if the bike had been through an accident serious enough to bend its huge main chassis, there should be other signs of the accident, and there aren't any.
  I don't know what to think.

[Bixxer Bob]

At first glance, I'm scratching my head here,  a frontal impact would ripple the underside of the tube, not the top.  I can't think what would do that. 

Edit,  unless it was bent under a frontal impact and what we re looking at is the heating and hammer work to knock it straight....

[Sin_Tiger]

Hard landings after wheelies  possibly off road work. I would have a close look at the underside, crack check by dye penetrant if possible.

Have a look at the rear sub frame just forward of where the rear rack should mount, common failure.

[Mustang]

That appears to be cracked ,split OEM welds from the gussets and ridden for quite a while like that which caused the frame to buckle /deform. It's actually a common failure for steamers that are subjected to Rough roads and hard use .

As for the frame geometry it's probably not at factory spec any more . Like Sin says check the sub frame on the rear for similar damage , they usually break there too from the same washboard rough road use and being ridden hard /overloaded.
Me myself I would find another frame from a breaker yard .

[Solon]

At first glance, I'm scratching my head here,  a frontal impact would ripple the underside of the tube, not the top.  I can't think what would do that. 

Edit,  unless it was bent under a frontal impact and what we re looking at is the heating and hammer work to knock it straight....

I think you scratched your head in the right direction, I checked underneath, and it is also rippled like a frontal impact bent the fork together with the front end downwards. There is no accident or wheelie or anything that could bend the chassis both downwards and upwards, so one of them is handmade in an attempt to straighten and diminish the damage. I looked at the fork closely. Unlike the rest of the bike which shows few sign of being used, the fork does look "old" and pretty worn out at those two inches where the oil seals rub most of the time. So I suppose the fork was changed with an older one from a breaker, and it was changed because of a frontal impact that destroyed it and bent the chassis downwards, only to be straightened up lateer at the cost of those ripples above.
The bike seems to handle well, but that chassis is certainly not trustworthy so I'll try to look for one in a better shape. Otherwise the bike seems in great condition and I still don't regret getting it. I guess that  a 2000 first registered bike with 36 000 km (not miles!) on it is still a bargain at the 700 Euros I paid for it.
Gonna check for frames now...