bubbling toilet during clothes washing

My house is about 45 yrs old. Over the years I had bubbles come up thru the toilet-no soap. Seemed to stop after replacing toilet w/ pressure flush type.a few years ago. About 7 weeks ago, toilet overflowed in opposite(east) side of house, water came up into tub also-after clean flush-no sewage. Had plumber snake it out from roof vent-nothing came back on snake. Last week, found out some bubbling had been occurring-dont know when?but overflow occurred again(east) Snaked again-about 70 ft. Nothing on snake. Had video run from both roof vents, found 1.) Flakes of rust and 2). What appeared to be a *sag* in the line....marked two spots indicated by fiber optic which happen to be in the garage about 6 ft apart.! Some kind of tie ins I guess? Rinsed the line for some time ,said might take care of problem for 6mo to yr- if rust flakes were the problem and are gone. Supposed to call me but didn't. No backups last week BUT today when washing clothes, bubbles in the toilet(WEST) closest to washer.No repeat back up in either side so far. Plan on calling but wondered if the bubbles don't really indicate a major problem. Can I get some more time out of this? BTW, the washer standpipe drain and bath sink drain are about 4 feet apart-wall between-this proximity a possible issue? The thing is going from east to west is just weird!!! How can cast Iron SAG?? confused scared and broke!!! :-) thx
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replying to Donn, Iggy wrote: I think you've got to rip open the garage wall or ceiling (wherever the pipe is) and investigate that sag. This will let you see what the plumbing actually is and if it even looks professional or proper or something of a patchwork of methods and materials.
Your home should be too young for Lead Drains, but that stuff is notorious for sagging and there could be a section of it used for whatever reason. However, you should be full cast iron and you may simply have a poorly done joint or a joint that was unsupported and has slowly fallen or settled out-of-level.
If shoddy material or work is found, then you may have other issues contributing to the overall problem...you may not have proper venting and especially for anything that's more than 5-feet from the vent stack pipes. S-traps or double-bendy pipes under sinks or other places usually indicate the lack of a vent.
One common contributor is the pipe immediately under the toilets. The cast iron there is bathed in both water and air and rusts itself shut. The toilets need to be removed and the rust chiseled off to return the pipes to full diameter (you must vacuum-out the chiseled rust).
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