I have a 04 Sienna that the motor has possibly gone bad in for the second time. Sludge has been a possible reason except this time no oil loss, no over-heating, no warning lights, just a loss of power and then a knock in the top of the egine...does anyone have any ideas as to why this has happened?

Some Toyoya engines are known for having sludge problems from what I have heard. Some engines are just more prone to engine sludging due to hot spots. The best way to prevent it is using a good high quality engine oil, regular change intervals and to obtain the best life indicator of your oil start performing UOA's (used oil analysis).