Avoiding detection? - early 1943

[bsalyers]

Hi, all;
I tried to find a relevant topic to post this under; some were close, but if there's one that's perfect, I didn't see it.
I'm in a big ol' Type IX in early 1943, playing at 97% realism, unmodified, latest patch applied. On my way to patrol off South Africa, I interecepted a large, fast (13 knots) convoy at night. I've tried my approach twice and have been detected both times. Once, I waited at periscope depth just barely abeam of the lead DE approaching from 270 degrees. He was still several thousand meters away when the pinging started. He had no trouble detecting me. Second try, I went to 25 meters and ordered silent running. Same thing; they started pinging me like the sun was out and the water was transparent. Is there a good way to approach these guys? How are they detecting me? They can't be randomly pinging as they go; I'd hear it. I'd try a surface approach, but from my experience and everything I've read in these forums, they'd spot me even more readily. I don't mind difficulty, but impossibility is a bit of a drag . Any suggestions?
TIA.