@barbarahayes the pulsating is most commonly caused by a clogged filter. You can either wash it out or replace it. I've tried the washing and failed. I ended up replacing mine which stopped the pulsating competently. Check the first 2 min of [https://www.youtube.com/watch?v=UafhshE0Th8|this video]
Depending on how much debris is already inside our Dyson, you may also have to look into a total disassembly to clean it out. Use these guides [[Topic:Dyson DC58]] to work on it.
Try the filter first.