Have had 740 B200e 5 speed manual injection for last 3 years.
Same problem on hot days above 70f usually more noticable in heavy traffic congestion in London.
Car leap frogs when overheated & fuel pump noise gets noisier than normal.
Car bought at 187k miles & is now over 231k miles & problem still persists, but no worse than when 1st discovered.
Found out that possible cause is that when tank less than half full, the pump in tank pulls in air causing break in fuel feed to injectors and this causes other pump outside to make a funny noise due to running dry temporarily.
Have found that keeping tank filled above at least 3 quarters on hot days will cure this problem although not entirely.
Have been advised due to age of 1988 740, it is better left alone untill tank pump gives up, after all its been perfectly fine for the last 44k miles or so.