Toyota Starlet what is this knock?

Hi, I've been directed to this section and wonder if there is anybody out there with some advice. My car has developed a knock like someone hitting 3 or 4 times with a hammer, back rear over wheel area. Fine over bumps, around corners, happens on the straight eg main road at steady speed. Had breakdown mad out, jacked it up, checked engine, did test run, said car brilliant. Did not have knock again until I did another long run. Seems to happen when car warm. Am doing unusual amount of necessary long journeys at the moment and happens after at least 40 miles. A few days ago did very long run and happened twice always same number of knocks. I crept home worried, at a crawl. Took into dealer, who jacked up, did visual, said fine safe to drive and not to worry unless it became worse. My car is well maintained, I know it's foibles. This knock is really loud but does not affect running of car but I am so concerned. Have checked all I can for anything loose and nothing. On return from a house hunt, it happened twice in a row on straight road travelling at 50 mph. With a few distant thumps, sounds a bit like a fly wheel rotating and knocking. The dealer thinks I am nuts I am sure and only offers my car a long visit at the garage to do runs out. He won't hear this knock since it happens only after such long runs and big spaces of mileage between. I am petrified something dire will go wrong at speed despite everyone saying it's fine. Any ideas? My car is well maintained. Have had since 2000, never had brakes or new exhaust. Always passes MOT, serviced until recently. Drive very carefully and always brake slowly. This has everyone perplexed. I do have some corroded brake pipes though and my one bumper and wheel rim at the back looks like someone may have knocked it, although everyone who has looked has picked nothing up. I am not a paranoid female this is a big noise. Any help any one can offer to give me fire power at the dealer will help. They are being most unhelpful. Thanks so much. Another big journey coming up and I'm a bit reticent.